If we're being realistic, I'd say Escape Plan or Little Deviants. Both are Vita-exclusives (which I think is needed to entice existing PS+ PS3 users to buy a Vita) and both have been out for awhile.
Another possibility would be LBP Vita since LBP2 is there for PS3 and it's been stated that it'll probably never leave since it's a big draw. That being said, LBP Vita is only a few months old, so it may be awhile.
On a related note, there's a rumor that Mortal Kombat will be next week's IGC addition for Vita, matching EU. If we're going to be given a remake, I for one would much rather have the MGS HD Collection. If the US PS+ really wants to outdo the EU PS+ like they claim, that'd certainly do it.
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