First I want to say, I have tried openwrt on my netgear wndr3800 router and my pc, the problem is same.
what I did:
https://downloads.openwrt.org/releases/21.02.1/targets/x86/64/openwrt-21.02.1-x86-64-generic-ext4-combined.img.gz for my pc and install it.
https://downloads.openwrt.org/releases/21.02.1/targets/ath79/generic/openwrt-21.02.1-ath79-generic-netgear_wndr3800ch-squashfs-factory.img for my wndr3800 router and install it.
after the fresh installation,
opkg install vsftpd
issue "service" to list all service status.
vsftpd is on status "stopped".
I try all, but vsftpd is still stopped.
then on other pc, issue "ftp 192.168.1.1", an error "500 oops: child died".
this problem is all on wndr3800 and my pc with openwrt.
then I tried openwrt 20.02.0 version, same problem.
then I tried openwrt 19.07.8, after issue those same commands, vsftpd work fine.
what I have tried is to install openwrt, issue same commands, I do not change vsftpd.conf.
So can we say, on openwrt 21.02.x version, there is a bug about vsftpd?