Well, looking at the man page at
http://vsftpd.beasts.org/vsftpd_conf.html, I read the following:
guest_enable
If enabled, all non-anonymous logins are classed as "guest" logins. A guest login is remapped to the user specified in the guest_username setting.
Default: NO
So it appears that is what is happening. All regular (read: non-anonymous) logins are being re-mapped to guest_username. So it appears you'll need to disable guest_enable and find an alternative method to allow "virtual" users.