FatBoy SoundFont weights 300+MB, so from what I understand it's not really an emulation but full instrumental sound samples package.
I've checked options in the vita port (latest SVN version) and following settings are not present:
Audio -> Preferred Device -> FluidSynth

Midi -> GM Device -> FluidSynth

If implementing it is possible on Vita without breaking the framerate I'd like to request this feature.