because his senate seat will go den
Not initially - PA has a Republican governor. He would name a replacement in 2012, and the seat would be up in a special election.
But I don't buy into the VP-gives-the-state stuff. Toomey was just elected two years ago, and I bet half of PA doesn't even know he's their senator (I live in PA).
The VP pick should be based on merit, not on geography - that rarely works.