It's just a rumor. The cause of it, is because of the hidden MGS V logo. They've never used roman numerals prior to that, so people are speculating that the V stands for Vita. It's kind of plausible, considering the Vita is capable of running fox engine (though I highly doubt it would look that good), we haven't even seen much of Ground Zeroes yet which is supposed to be a prequel to MGS 5- so it wouldn't make a lot of sense to reveal the fifth game yet, it doesn't seem like it would make sense to release two MGS game prior to MGS 5 either... But to be honest, the whole game is a mystery, other than the fact that it is somewhat obviously connected to MGS. I mean, the main character looks exactly like Naked Snake. Another possibility is that Ground Zeroes and The Phantom Pain are just the same game, but if so, why hide The Phantom Pain's identity as a MGS game?