Sony announced a bunch of games for its PlayStation Vita at E3 this year, but a port of Marvel Vs. Capcom 3 was definitely not among them. You'd remember if it was, largely because that's pretty awesome news. A port may be happening though, if a now-removed listing on Amazon France is to be believed.
The product page was found, or at least tweeted, by Gamertag Radio co-host Danny Pena (via Joystiq). You can still follow the link if you'd like to satisfy your curiosity, but trust me: it's dead. The listing didn't appear to be for the upcoming "Ultimate" version of the game either, which further suggests that its appearance was simply an accident.
That's not to say a port isn't coming eventually, but with Ultimate MvC3 out in November and the Vita pushed to "early 2012," the Amazon listing is a little premature to be real.