Passive FTP that would work inside and outside of NAT
Solution 1:
There some alternative solutions:
If possible in vsftpd, use different config for LAN and other. May be two different instance of vsftp.
Add src-nat for dst-nat connections on router internal ip
Use for server different from internal clients IP subnet.
PS. I quickly look at vsftpd config manual. It not possible to use different configuration for different client IP.