While the Cortex-A9 architecture can go up to 2 GHz, the specific implementation of the Cortex-A9 in the PS Vita does not; it only goes up to 444MHz. ARMv7 is the instruction set; I suppose that was unclear. I'm not clear if this is a limitation imposed by the hardware or the kernel, but from usermode, attempts to clock it higher fail. Wikipedia, as it is wont to be, is incorrect in this case, and the citation is from preview hype, not from an actual source.
The CPU on the N3DS is slightly more powerful than the Vita's, at 804MHz but with an older architecture (ARM11 MPCore), leading to mGBA on N3DS edging out the Vita, since it's CPU-bound. While the cores on the Vita are more generally accessible (N3DS gives you 2 core and time slices on the others, whereas the Vita outright gives you three), mGBA is not properly multithreaded, so the core count is mostly irrelevant. The O3DS runs at 268MHz, so it's at a clear disadvantage in all cases. Trust me, I was surprised when I found mGBA Vita running slower than mGBA 3DS before clocking the Vita to 444MHz (instead of 333MHz) was discovered.
