well if you had a .sql file of your db it would contain all the tables and their seperate datas in sql queries, so bascially all you do is copy and paste the sql query for your user and session tables and run them in phpmyadmin's query window
hmm not sure, probably since it was a backup done by cpanel while it was reuploading it skipped over it or it was never backed up in the first place or an error occured when reinserting those tables and it just continued on reuploading , not inserting those 2 tables. i would also go over to vbulletin.com and find the full list of tables your susposed to have(wana make sure your not missing any others)