GO THERE:
http://www.vbulletin.com/forum/forumdisplay.php?f=92 it is the official forum for IMPORTS
1- you upgrade both sites to the latest version, for security and speed
1a- put all attachments in database instead of files, for compatibility and ease
2- you install both forums in the same sql server
3- IMPORT the older one or the smaller one into the other.
never use sql functions to merge both. the ImpEx system from vBulletin will take care of everything.