Basically, I need to integrate vBulletin within my Asp.net application. A user logged in to the asp.net website, should not be asked to enter credentials, when accessed vBulletin.
I don't know of any .net vBulletin hacks or mods that will authenticate users against a foreign database. If there was one available we would probably be using it ourselves. Even their Facebook integration creates a new vB user, it doesn't authenticate (after the first time) using the Facebook login.
You should be able to utilize vBulletins forms authentication with your own web application. The easiest way is to upload your web application to the same subfolder where vBulletins has its website is also protected by its forms authentication.