Just so you know A4O and FFA aren't really spin offs. The story was 100% cannon, and therefore they are booth a part of the series. The gameplay was just totally different, this does not make them spin offs.
The only thing wrong with FFA was that it was short really. Hardly makes it a let down does it? I think it's a great game (I have only played the trial though)
The tower defense elements seemed to work with most fans, but I'm sure it wont stick around for the next game. It was just a good little change for a good little game.
As for Insomniac on Vita? Insomniac say they have no plans at all for touching the Vita. If Sony want a Vita version they get another developer to do it, because Insomniac don't have the ability to work on Vita, as they are fully focused on creating big games for the PS3. So Sony are getting Tin Giant to do it. Insomniac are making the game, Tin Giant are making it run on the Vita, simple as that.
Go and play some LittleBigPlanet, it's good for you!