[Attilitus]

Go to vbulletin options, then to User Registration Options.

Then find Username Regular Expressions. Click on the "help button" in order to view the REGEX codes which allow certain characters.

I will go ahead and paste the contents of the helpbutton.

^[A-Z]+$ - Characters from A-Z only
^[A-Z ]+$ - Characters from A-Z including space
^[A-Z0-9 ]+$ - Alphanumeric characters including space
^[\x20-\x7E]+$ - ASCII characters from 32-127

So just choose the allowed characters that you want in usernames, and paste that into the Username Regular Expressions option field in vbulletin options under User Registration Options.