In a recent interview with IGN Gearbox Software's Randy Pitchford discussed the idea of Borderlands 2 on PS Vita.
He was especially intrigued by the notion of having cross platform saves between PS3 and PS Vita - but also said that a lot of things had to come together for it to become a reality. First, since Gearbox are busy with other projects they would need to find a developer to partner with that they trust, second publisher 2K Games have to keen on it, and thirdly Sony needs to be interested in the idea.
Obviously, we're far away from Borderlands 2 hitting PS Vita. But at least one of the key individuals involved is interested.