Edit the VsFtpd configuration file:
vi /etc/vsftpd.conf
and modify as:
userlist_deny=NO userlist_enable=YES userlist_file=/etc/vsftpd.allowed_users
In the file /etc/vsftpd.allowed_users add the username of the users that can login.
The not allowed users will get an error that they can't login before they type their password.