Now it's time for someone to come along and say that I don't know what I'm talking about and this whole post is moot (or makes me looks dumb).....
GrindedDown is right. It would be OK for some basic shapes / UI elements to be vector-based, but complex graphics are too much of a load for the CPU.
I can't imagine having to load tens of icons that look like this:
every time you hit your home button. The iPad would probably become unresponsive / laggy for a couple seconds and drain battery like crazy.
Quick example of the kind of issues you'd get: Earlier I made a simple button, to be shown above a live camera view. It's just a white button, with a simple shape, and a transparent background. I made it with vectors so it's easy to make retina/standard versions. But it's white, and when it's shown over a white background it becomes invisible - so I add a slight drop shadow to make it visible. Drop shadows are very hard to render with vectors. The only options are to save it as a bitmap, or to remove the drop shadow and add a black outline instead - which doesn't look anywhere near as good.