I would expect it to be available at least in the same sense that it is "available" for PSP - PS1 classics and minis (though it requires a PS3 to get plus in the first place). Beyond that, like glaciusx25 said, none of us know.
I agree. To appeal to the widest audience possible, the PSminis and PS1 classics will be compatible with the PSP and PS Vita for now. I would expect over time PS+ will integrate PS Vita content into the subscription service...
To some extent I think it has to be to keep from alienating the existing PSP audience. I can already go to the store and download my PS+ PSOne and Minis offerings, so I do not see Sony taking that away from PS+ subscribers.
Also, there have been conversations about cloud saves, so Sony either has to say all Vita users get cloud saves or keep them as a PS+ specific feature in order to drive PS+ subscription adoption. Of course I am not sure how many Vita owners that do not currently subscribe to PS+ will want to pony up for PS+ in order to take advantage of cloud saves.