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.