2 domains 1 database 1 sub forum
 

With vbulletin would it be possible to have 2 different domains but one database.

With site 1 having the main forum and site 2 just have a subforum displayed but with a different header?

if forum on site 2 is just a subforum of the main forum, then it's ok

Yes the forum on site 2 is a subforum of the main forum on site 1 but i need it to have a different header.

just add another styleset making the different header and the in the forumoptions of your admin cp, you can assign the style to this subforum

how do you output a subforum onto another domain?

http://www.talkloud.net/ <-- Main Forum
http://www.banemacleod.com/ <-- Subforum styled to look like another site. If you click comment you will notice it is actually at talkloud, simply because I was too silly to do it properly but it illustrates it can be done quite well if this is what you mean.