In that case it should be fairly easy to do - The only thing that you may need to watch out (I've never attempted this myself with vB 3.5 or 3.6

) for is how the password is stored in vBulletin, as it uses an md5 hash with a unique salt; Looking through how the registration file does it should tell you how that is created but if there are any questions, you are likely to find someone who knows here
Chris