In terms of hardware, Vita has 4 ARM cores(up to 2.0ghz), vastly more powerful than the 2 268mhz 3DS ARM cores. It has much more memory and a quite faster gpu. Also it has touch controls and all other 3DS bells and whistles.
Since both the Vita and the 3DS use ARM cores, the cpu emulation performance hit will be low.
So IF and i say IF both consoles are hacked, there is the possibilty of a 3DS on Vista emulator. And it will be quite fast.
Remember that the original PSP was able to emulate the PS1 easily, because it used the same architecture of CPUs, MIPS. It could also emulate the N64, because the N64 also used a MIPS cpu. If only the PSP had more ram and a tad faster cpu, it could have been a wonderful N64 emulation machine. But the PSP couldn't emulate the DS even though it is not that much more powerful, and actually was weaker than the N64.