Display Name
 

Does anyone know a mod that allows people registered to my vbulletin forums pick a username then a Display name?

You can do this with by defining a new User Profile field via the Admin CP.

put how can i get it to display instead of there username?

Depending on where you want it, it could be just a simple template edit (such a post) or you'll have use hooks to pull in the userfield in other places that don't normally fetch it when displaying username.

As was said this is simple if you only want this to show in the postbit... more complicated but not impossible if you want it elsewhere... but it may get confusing because if people need to use PM's they would have to send them to their username, not display name... also 'quotes' may be an issue quoted by username by default.