SquareEnix will be on the Vita. They're just slow to arrive on any system that requires budgets and dev time. Their first major PSP game was Crisis Core and remember how it was vaporware and took years to come out? They had a bunch of DS games in the mean time like TWEWY, Rocket Slime 2, DQ remakes, FF3r. Then after CC, they started pumping out more stuff and eventually got Dissidia, T3B, BBS, Type 0.
I expect to see a lot of the big Japanese players on Vita, but like the first year or two of the psp, I expect Vita to be ports and low effort games.
btw, KH is on 3DS because Nomura doesn't give a shit about system wars. After BBS, he wanted to make a KH game in < 2 years with the BBS engine and team and at the time, 3DS was the only option (unless he wanted it on PSP again, but no one knew if the PSP would still be alive in 2012), so it was on 3DS. After KH 3D, he'll figure out where the next one will go. KH has been on GBA/DS/PSP/3DS. It'll probably be on Vita too.