Well, the Vita Plus isn't already on top of your existing subscription, from my understanding. You have to buy the Vita Plus separately, for the same price as the PS+, which I don't think makes sense.
As much I defend Plus, and the Vita against ridiculous claims of how weak the console is in terms of content, I'm not sure the Vita has been around long enough to have enough back-content to offer people via Plus. And for the future, the Vita is seeming to be more and more poised as an addition to the PS3 experience, and not its own.
That being said, as soon as I see a month for Vita that seems worth it to me, I'll hop on board, because Plus has been so good thus far.