I think Rick Perry would be a good choice. Santorum may need help in the South, and Perry could help with organization and money. And he will help with Hispanic voters.
Susana Martinez might be a good pick, but she would have to be well vetted first.
Perry is an embarrassing gaffe machine who will never get out of Texas again, and whose epic fail of a presidential campaign was so bad that it put his day job in jeopardy. Also, the last thing Santorum (or Romney for that matter) needs is a VP who reminds people of Dubya or Palin, and Perry combines the worst characteristics of both.
I'd take Dubya in a heartbeat over this crop of candidates, except maybe Mittens only because I think he has a clue at fixing the economy.