I like Santorum but he brings absolutely nothing to the ticket. Politically, his best match would be a female running mate or an African-American. Demographically, that brings strength to the ticket.
The ticket will be shared with someone who can deliver either a swing state necessary to a win, or someone who can deliver a large populated state which yields meaningful delegate numbers, or someone who can cap off a particular region where the nominee may need a boost.
The need will unfold over time, but Newt may need Pennsylvania because it is generally purple, making it a swing state. And Texas has increased delegates this year, enough to make up for NOT being on the dang ballot in Virginia. Santorum or Perry may be useful on the ticket along with others.