[Opserty]

The problem with this is vBulletin uses usernames for things other then display. For example, to send PMs you use the username (then vBulletin goes and looks up the userid to send to). The last post info is also cached and the username is stored in that cache. If I remember correctly the build_new_post() function in functions_newpost.php might be where it updates the last post info. But I'm not 100% sure. You'll need to have a fair bit of knowledge of PHP as it won't be a case of simple template replacements.

p.s. You may be better of modifying vBulletin to use the email field in the way in which you need it. Instead of trying to replace the usernames. I think there is a modification out that, that will allow users to log in with their email.