OK, all redacted. In the example 2001:db8:f00f:100::/56 would be the prefix I would be routing (obviously an example). The prefix I'm using (not the example) worked fine with the same router with the same configuration running Chaos Calmer.
Interestingly, I've discovered that if I manually set a default ipv6 route after the VPN tunnel is up, nothing changes, but around 30 minutes later all of my systems start getting routes distributed to them. If I could shorten that refresh to less than a minute, I could probably kludge that into my startup scripts as a sloppy workaround.
Thank you for your help. 
Here's my redacted config:
{
"kernel": "4.14.209",
"hostname": "site-router",
"system": "Atheros AR9344 rev 2",
"model": "NETGEAR WNDR3700v4",
"board_name": "wndr3700v4",
"release": {
"distribution": "OpenWrt",
"version": "19.07.5",
"revision": "r11257-5090152ae3",
"target": "ar71xx/nand",
"description": "OpenWrt 19.07.5 r11257-5090152ae3"
}
}
package network
config interface 'loopback'
option ifname 'lo'
option proto 'static'
option ipaddr '127.0.0.1'
option netmask '255.0.0.0'
config interface 'lan'
option ifname 'eth0.10'
option force_link '1'
option type 'bridge'
option proto 'static'
option ipaddr '192.168.2.1'
option netmask '255.255.255.0'
option ip6addr '2001:db8:f00f:2::1/64'
option ip6gw '2001:db8:f00f:2::1'
config interface 'wan'
option ifname 'eth0.1'
option macaddr '00:00:5e:06:06:06'
option proto 'dhcp'
config interface 'wlan'
option proto 'static'
option ipaddr '192.168.3.1'
option netmask '255.255.255.0'
option broadcast '192.168.3.255'
option ip6hint '3'
option ip6assign '64'
option dns '8.8.8.8'
option ip6addr '2001:db8:f00f:3::1/64'
option ip6gw '2001:db8:f00f:3::1'
config interface 'wlan2'
option proto 'static'
option ipaddr '192.168.4.1'
option netmask '255.255.255.0'
option broadcast '192.168.4.255'
option ip6hint '4'
option ip6assign '64'
option dns '8.8.8.8'
config interface 'sdmz'
option ifname 'eth0.12'
option type 'bridge'
option proto 'static'
option ipaddr '192.168.1.1'
option netmask '255.255.255.0'
option broadcast '192.168.1.255'
option ip6assign '64'
option ip6hint '1'
option ip6addr '2001:db8:f00f:1::1/64'
config switch
option name 'switch0'
option reset '1'
option enable_vlan '1'
option enable_vlan4k '1'
config switch_vlan
option device 'switch0'
option vlan '10'
option ports '0t 1 2 3'
config switch_vlan
option device 'switch0'
option vlan '12'
option ports '0t 4'
config switch_vlan
option device 'switch0'
option vlan '1'
option ports '0t 5'
config interface 'vpnc0'
option ifname 'tun0'
option proto 'none'
option auto '1'
option ipv6 '1'
option force_link '1'
option ip6prefix '2001:db8:f00f:100::/56'
option ip6addr '2001:db8:f00f::1000/64'
package wireless
config wifi-device 'radio0'
option type 'mac80211'
option channel '11'
option hwmode '11g'
option path 'platform/ar934x_wmac'
option htmode 'HT20'
config wifi-iface
option device 'radio0'
option network 'wlan'
option mode 'ap'
option ssid 'SSID1'
option encryption 'psk2'
option key 'REDACTED'
config wifi-device 'radio1'
option type 'mac80211'
option channel '36'
option hwmode '11a'
option path 'pci0000:00/0000:00:00.0'
option htmode 'HT20'
config wifi-iface
option device 'radio1'
option network 'wlan2'
option mode 'ap'
option ssid 'SSID2'
option encryption 'psk2'
option key 'REDACTED'
package dhcp
config dnsmasq
option domainneeded '1'
option boguspriv '1'
option filterwin2k '0'
option localise_queries '1'
option rebind_protection '0'
option rebind_localhost '1'
option local '/lan/'
option domain 'example.com'
option expandhosts '1'
option nonegcache '0'
option authoritative '1'
option readethers '1'
option leasefile '/tmp/dhcp.leases'
option resolvfile '/tmp/resolv.conf.auto'
option localservice '0'
config dhcp 'lan'
option interface 'lan'
option start '100'
option limit '150'
option leasetime '12h'
option ra 'server'
config dhcp 'wan'
option interface 'wan'
option ignore '1'
option start '30'
option limit '169'
option leasetime '12h'
config dhcp 'wlan'
option interface 'wlan'
option start '30'
option limit '464'
option leasetime '12h'
option ra 'server'
list dhcp_option '6,192.168.1.1'
config dhcp 'wlan2'
option interface 'wlan2'
option start '30'
option limit '464'
option leasetime '12h'
option ra 'server'
list dhcp_option '6,192.168.1.1'
config dhcp 'sdmz'
option interface 'sdmz'
option start '30'
option limit '464'
option leasetime '12h'
option ra 'server'
option dhcpv6 'server'
config odhcpd 'odhcpd'
option maindhcp '0'
option leasefile '/tmp/hosts/odhcpd'
option leasetrigger '/usr/sbin/odhcpd-update'
config host
option mac '01:00:5e:ff:44:77'
option name 'test-system'
option ip '192.168.1.2'
package firewall
config defaults
option syn_flood '1'
option input 'ACCEPT'
option output 'ACCEPT'
option forward 'ACCEPT'
config zone
option name 'lan'
list network 'lan'
list network 'sdmz'
option input 'ACCEPT'
option output 'ACCEPT'
option forward 'ACCEPT'
option log '1'
config zone
option name 'wlan'
list network 'wlan'
list network 'wlan2'
option input 'ACCEPT'
option output 'ACCEPT'
option forward 'ACCEPT'
config zone
option name 'wan'
list network 'wan'
option input 'REJECT'
option output 'ACCEPT'
option forward 'ACCEPT'
option mtu_fix '1'
option conntrack '1'
config forwarding
option src 'lan'
option dest 'wan'
config forwarding
option src 'wan'
option dest 'lan'
config forwarding
option src 'wlan'
option dest 'wan'
config forwarding
option src 'wan'
option dest 'wlan'
config rule
option name 'Allow-Ping'
option src 'wan'
option proto 'icmp'
option icmp_type 'echo-request'
option family 'ipv4'
option target 'ACCEPT'
config rule
option name 'Allow-IGMP'
option src 'wan'
option proto 'igmp'
option family 'ipv4'
option target 'ACCEPT'
config rule
option name 'Allow-ICMPv6-Input'
option src 'wan'
option proto 'icmp'
option limit '1000/sec'
option family 'ipv6'
option target 'ACCEPT'
config include
option path '/etc/firewall.user'
config rule
option src 'wan'
option dest 'lan'
option proto 'esp'
option target 'ACCEPT'
config rule
option src 'wan'
option dest 'lan'
option dest_port '500'
option proto 'udp'
option target 'ACCEPT'
config rule
option target 'ACCEPT'
option src 'wan'
option proto 'tcp'
option dest_port '555'
option name 'CN-SSH'
config rule
option name 'Allow-WAN-DNS-UDP'
option src 'wan'
option proto 'udp'
option dest_port '53'
option target 'ACCEPT'
config rule
option name 'Allow-WAN-DNS-TCP'
option src 'wan'
option proto 'tcp'
option dest_port '53'
option target 'ACCEPT'
config rule
option name 'Allow-WLAN-DNS-UDP'
option src 'wlan'
option proto 'udp'
option dest_port '53'
option target 'ACCEPT'
config rule
option name 'Allow-WLAN-DNS-TCP'
option src 'wlan'
option proto 'tcp'
option dest_port '53'
option target 'ACCEPT'
config rule
option name 'Allow-WAN-DHCP-Request'
option src 'wan'
option src_port '67-68'
option dest_port '67-68'
option proto 'udp'
option target 'ACCEPT'
config rule
option name 'Allow-WLAN-DHCP-Request'
option src 'wlan'
option src_port '67-68'
option dest_port '67-68'
option proto 'udp'
option target 'ACCEPT'
config rule
option name 'Allow-WAN-DHCP-Response'
option dest 'wan'
option src_port '67-68'
option dest_port '67-68'
option proto 'udp'
option target 'ACCEPT'
config zone 'vpnc'
option name 'vpnc'
option network 'vpnc0'
option device 'tun0'
option input 'ACCEPT'
option forward 'ACCEPT'
option output 'ACCEPT'
option masq '0'
config forwarding 'vpnc_forwarding_lan_out'
option src 'lan'
option dest 'vpnc'
config forwarding 'lan_forwarding_vpnc_out'
option src 'vpnc'
option dest 'lan'
config forwarding 'vpnc_forwarding_wlan_out'
option src 'wlan'
option dest 'vpnc'
# This file is interpreted as shell script.
# Put your custom iptables rules here, they will
# be executed with each firewall (re-)start.
# Internal uci firewall chains are flushed and recreated on reload, so
# put custom rules into the root chains e.g. INPUT or FORWARD or into the
# special user chains, e.g. input_wan_rule or postrouting_lan_rule.
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 state UNKNOWN qlen 1000
inet6 ::1/128 scope host
valid_lft forever preferred_lft forever
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 state UP qlen 1000
inet6 fe80::b0db:6aff:fe22:c61d/64 scope link
valid_lft forever preferred_lft forever
5: br-lan: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 state UP qlen 1000
inet6 2001:db8:f00f:2::1/64 scope global
valid_lft forever preferred_lft forever
inet6 fe80::120d:7fff:fe4d:56f6/64 scope link
valid_lft forever preferred_lft forever
7: br-sdmz: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 state UP qlen 1000
inet6 2001:db8:f00f:1::1/64 scope global
valid_lft forever preferred_lft forever
inet6 fe80::b0db:6aff:fe22:c61d/64 scope link
valid_lft forever preferred_lft forever
9: eth0.1@eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 state UP qlen 1000
inet6 fe80::120d:7fff:fe07:707/64 scope link
valid_lft forever preferred_lft forever
10: wlan0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 state UP qlen 1000
inet6 2001:db8:f00f:3::1/64 scope global
valid_lft forever preferred_lft forever
inet6 fe80::120d:7fff:fe4d:56f6/64 scope link
valid_lft forever preferred_lft forever
11: wlan1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 state UP qlen 1000
inet6 fe80::120d:7fff:fe4d:56f8/64 scope link
valid_lft forever preferred_lft forever
12: tun0: <POINTOPOINT,MULTICAST,NOARP,UP,LOWER_UP> mtu 1500 state UNKNOWN qlen 100
inet6 2001:db8:f00f::1000/64 scope global
valid_lft forever preferred_lft forever
inet6 fe80::2684:a677:5dd5:7e01/64 scope link
valid_lft forever preferred_lft forever
2001:db8:f00f::/64 dev tun0 metric 256
2001:db8:f00f:1::/64 dev br-sdmz metric 256
2001:db8:f00f:2::/64 dev br-lan metric 256
2001:db8:f00f:3::/64 dev wlan0 metric 256
2000::/3 dev tun0 metric 1
fe80::/64 dev eth0 metric 256
fe80::/64 dev br-sdmz metric 256
fe80::/64 dev eth0.1 metric 256
fe80::/64 dev br-lan metric 256
fe80::/64 dev wlan0 metric 256
fe80::/64 dev wlan1 metric 256
fe80::/64 dev tun0 metric 256
local ::1 dev lo table local metric 0
anycast 2001:db8:f00f:: dev tun0 table local metric 0
local 2001:db8:f00f::1000 dev tun0 table local metric 0
anycast 2001:db8:f00f:1:: dev br-sdmz table local metric 0
local 2001:db8:f00f:1::1 dev br-sdmz table local metric 0
anycast 2001:db8:f00f:2:: dev br-lan table local metric 0
local 2001:db8:f00f:2::1 dev br-lan table local metric 0
anycast 2001:db8:f00f:3:: dev wlan0 table local metric 0
local 2001:db8:f00f:3::1 dev wlan0 table local metric 0
anycast fe80:: dev br-sdmz table local metric 0
anycast fe80:: dev eth0 table local metric 0
anycast fe80:: dev br-lan table local metric 0
anycast fe80:: dev eth0.1 table local metric 0
anycast fe80:: dev wlan0 table local metric 0
anycast fe80:: dev wlan1 table local metric 0
anycast fe80:: dev tun0 table local metric 0
local fe80::120d:7fff:fe07:707 dev eth0.1 table local metric 0
local fe80::120d:7fff:fe4d:56f6 dev br-lan table local metric 0
local fe80::120d:7fff:fe4d:56f6 dev wlan0 table local metric 0
local fe80::120d:7fff:fe4d:56f8 dev wlan1 table local metric 0
local fe80::2684:a677:5dd5:7e01 dev tun0 table local metric 0
local fe80::b0db:6aff:fe22:c61d dev br-sdmz table local metric 0
local fe80::b0db:6aff:fe22:c61d dev eth0 table local metric 0
ff00::/8 dev eth0 table local metric 256
ff00::/8 dev br-lan table local metric 256
ff00::/8 dev br-sdmz table local metric 256
ff00::/8 dev eth0.1 table local metric 256
ff00::/8 dev wlan0 table local metric 256
ff00::/8 dev wlan1 table local metric 256
ff00::/8 dev tun0 table local metric 256
0: from all lookup local
32766: from all lookup main
4200000001: from all iif lo lookup unspec 12
4200000005: from all iif br-lan lookup unspec 12
4200000007: from all iif br-sdmz lookup unspec 12
4200000009: from all iif eth0.1 lookup unspec 12
4200000010: from all iif wlan0 lookup unspec 12
4200000011: from all iif wlan1 lookup unspec 12
4200000012: from all iif tun0 lookup unspec 12