It would probably be possible, but personally I think it would make some of the gameplay too difficult.
Maybe if it was for some minor stuff, side games, housing, chocobo stables, etc would be possible + like the smart phone apps they are making for FFXIV on Vita would most likely be possible but full on gameplay (battle, raids) would be too hard imo. Also say you had the content finder and you can select what platform they are on and you see people on "vita" I doubt much of them will get invited to battle related contents.

Btw didn't someone show a remote play version of FFXIV (current) on vita before?? I remember seeing something like that.