Well, afaik you wouldn't need to access the database again for the username and password as they should already be validated and have a cookie. This means that you only need to include the cookie function (where and whatever that may be) to check they are logged in, otherwise display a link so they can do a vB login and get the cookie required?
Also I think you need to make sure that the cookie is available to the entire domain, and not just vB.
Sorry i can't help more, but I think thats the approcahe you may need to take? Could someone confirm this or tell me I am wrong