I have had problems in the past with users using punctuation in their user names, such as O'Henry not being able to log in on the classifieds. I am using the vBulletin user base for registration, user names and passwords. My classifieds installation is on a different domain name, but it is on the same server and it works absolutely great!
In the vBulletin admin panel, User Registration Options, Username Regular Expression, I placed ^[A-Za-z0-9. -]+$
to prevent folks from registering with anything but numbers, letters, spaces, dots or dashes in their user names.
However, this does not prevent them from placing weirdness in their passwords. I just had a user with &, #, ! and % in his password. Wonderful for security and it works for logging into vBulletin...
BUT, this will not work for a user logging in on my configuration of PhotoPost classifieds, using the vBulletin user base on a different domain. As I said, this is posted just so you know.