I believe that N64 emulation is certainly possible, but whether it will run well for most games is more questionable. There are a few things to take into consideration. Firstly, N64 is more difficult to emulate than the PlayStation 1. Secondly, assuming the Vita's hardware is up to the task, software optimization and the type of emulation used is going to be a major factor in how well it will run.
I think it might be worthwhile to look at how N64 emulators run on hardware like the iPad 2 or iPhone 4S. If the emulators run well enough on those platforms then I think there might be some hope (but then again those devices are running at higher clockspeeds, and the Vita's additional cores are probably not going to be very useful since, if I am not mistaken, most N64 emulators, at most, scale to two cores and are more dependent on single threaded speed), but if the emulation is slow on those platforms then there is a good chance that the Vita version will not run much better (unless of course the emulators were not properly optimized).