what my post means is that the technique for verifying the password can not be replicated into vBulletin, and cannot be retrieved either... you have to deal with a "no solution" directly...

as you suggested, you could use the "Merge Users"... once the users register again, they contact you to tell you what was their username and what was their old email, so you can trace who they were, and then merge both users.. it's a simple "source user (old user without password)" >> "merge with target user"

something else would be to have a minimal form coded which would detect the old password and ask the user to enter a new password to update it...

something like:

.............username:
.........old password:
.......new password:
..confirm password:

.. i think of this, your forum was already updated etc ?!.. would be complicated if you do not have your old database though, because this technique need to verify the old password with the old forum dB... vBulletin do not keep the old password as it is not in a proper format...