They have a Thread on vbulletin.com which claims to have a solution for this issue. Check out:
This Thread is from
2001, so there may be an option now. Checking admincp also.
checking for illegal characters in username
http://www.vbulletin.com/forum/showthread.php?t=13190
In the last Post, it appears to have a solution the author claims was working for this purpose by editing the member.php template.
Regards,