User Profile Field replacing Username

[mcdull]

I would like to use a user profile field (which they can decide their screen name) to replace all username in vB.
How can I do it in the easiest way? Are there any replace function in vB and can I easily locate all the username field?

(For speical reason the username must be in certain format so that I have to use a custom field to replace the username in vB)

Thanks