vsftpd-3.0.0 made the listen mode default. In order to maintain backward
compatibility, default /etc/vsftpd.conf in SUSE contains

listen=NO
listen_ipv6=YES

if you plan to use xinetd (/etc/xinetd.d/vsftp) you don't need to change
anything by default, but if you have ipv6 available you have to set
listen_ipv6=NO too in order for xinetd to work.
Alternatively you can also use systemd socket service that listens on
port 21 and starts the server like xinetd would. This service does
not require you to change ANY variable in the /etc/vsftpd.conf.