The reason you use a chatbox/shoutbox on vbulletin is for the association to the user information database. If your are coming from another server will you already be authenticated with vbulletin? Not that I like the plan, but you could load vbulletin core and your chatbox in a specially crafted IFRAME on your main page.
Depending on what you are doing you might just want a 3rd party chatbox.