Even if you're right (you're not, gaming is great on iOS, but whatev). Even if you're right, it's not necessarily about whether someone would buy those devices just for games or choose those devices instead of a 3ds or Vita. As great as an iPad is for games, I have no doubt that the Vita is capable of more.
The much more common occurance will be people who already have iDevices who say, "Ya know, I want a Vita, and in the past I would have got one, but now I already have this pretty damned capable portable game machine with me at all times (iDevice). It's good enough and I just can't justify laying down $200+ when I've got this thing in my pocket already."