You should use listen_ipv6=YES instead of listen=YES in /etc/vsftpd/vsftpd.conf Version-Release number of selected component (if applicable): vsftpd-2.0.1-1
Parameters listen_ipv6 and listen are mutual exclusive. So like with listen parameters, listen_ipv6 vsftpd will listen on an IPv6 socket instead of an IPv4 one. Default conf file is eg. set to IPv4 adresses.
IPv6 sockets do accept IPv4 connections but IPv4 sockets cannot accept IPv6 connections.
You're right. Fixed in new release.