This is still all a bit obtuse:
Using LuCI to create a new interface name eth0_3, cover interface eth0.3, unmanaged:
This does nicely create eth0_3 in the LuCI CLI and it also creates eth0.3, but in UCI it is creating "config interface eth0_3", not configure "config device" as described in the openwrt VLAN manual page.
There is also no indication in LuCI what VLAN ID the interface actually uses. And "ip link" does not show it either. Other linux documentation says that "ip link" shows the vlan ID when using "ip link -d", but that option is not supported by openwrt it seems.
The vconfig command also does not seem to exist in openwrt.
Alas, the kernel is useful, "cat /proc/eth0.3" shows:
eth0.3 VID: 3 REORDER_HDR: 1 dev->priv_flags: 1001
Is there really no CLI command in openwrt that can show that an interface is a VLAN and show its VLAN ID ??
But overall, it seems to work as long as one understands the magic of creating and "covering" an interface "ethN.M" to create a vlan ID M on ethM.
How do i raise a wish for better LuCI GUI for something like this ?