actually this is completely different from the first request...

this is not a new request, many site owners request that kind of things... if the original script is easy to understand, registration and access can be easily bridged to vBulletin... if the engine is really strange -- i say really -- you could have to pay a good amount to have a really specific bridge for that... but i could say it's close to easy on most cases... if someone charge you more than 250$ for that, i would say they want to scam you...