Insomniac have been very evasive about Vita Nexus, refusing to confirm or deny its existence since the first hints before it even released on PS3, which makes me think that it's anther dreadful Q-Force style port that they want to push out without much fanfare. I hope I'm wrong since the game on PS3 is great and I normally like to think quite highly of Insomniac in terms of quality, but it's the only reason I can think of that would result in them not actually announcing a Vita version, while still refusing to outright deny it, even when directly asked.
I do wonder how Insomniac feel about awful ports like Q force Vita and Gladiator HD going out with their names on. I'd imagine that would cause a lot of tension between Insomniac and Sony, which could be part of the reason behind partnering with MS for Sunset Overdrive? I certainly hope that they weren't happy with the quality of those two examples anyway.