I don’t know if that is the most likely. Romney has spend millions of dollars publicly assassinating the character of Santorum. Why would he choose to play second banana to Mitt when he could team up with Newt and be at the top of the ticket. I actually think they would make a formidable team. Newt could play the unlikable but vicious and competent VP role ala Dick Cheney.
There are two reasons Rick would VP for Mitt:
1. Santorum + Gingrich < 1144
2. Santorum (despite his good points, which he has) is not really a conservative and has no real objection to Romney