The following error may occur on the client side: Fatal error: 500 OOPS: priv_sock_get_cmd if running an amd64 kernel with vsftpd-3.0.x.

To solve this bug you will need to add the following to your /etc/vsftpd/vsftpd.conf:

seccomp_sandbox=NO

Refer to https://bugzilla.redhat.com/show_bug.cgi?id=845980 and Gentoo Wiki

P.S. And just in case add next line to config file

allow_writeable_chroot=YES

for adequate working of virtual users.