vsftpd would refuse login with *any* username, giving the error message:
[SOLUTION]530: permission denied
The problem was, that the userlist option can either be used for denying access, or allowing access to all usernames listed in the "userlist_file" (usually "/etc/vsftpd_users").
In my case, the permission denied was caused by the following entry:
The default is "userlist_deny=YES", so uncommenting the above line solved my problem.userlist_deny=NO