[kh99]

I think you can create a user by inserting rows in the user, userfield, and usertext tables. In the latter two, the userid has to match but the rest of the fields can be the default values if you don't have data to fill them. But one issue I see is getting the password/salt fields set so that the user can log in to vbulletin. You'd either have to have the user's password in plain text so that you can hash it like vbulletin does, or else modify the vbulletin scripts that check the password to use whatever algorithm your main site uses.