Hi Lars!
I'm no expert, but I'll try to answer some of your questions. I actually have a TRITON at home right now. I'm trying it out before buying it.
So...
Yes, TRITON has hold for arpeggios.
Actually the whole arp. part of TRITON is amazing in my opinion.
The Vocoder is like any other effect (insert & master), with the difference that is takes two "effect-spaces". I've tried it very little, but one of the combo-sounds uses it and that sounds really great!
I have no idea how it is compared to other vocoders. The ones I've tried have been low-quality software-based.
TRITON has 64 notes of polyphony.
The MOSS board for TRITON is the same as for TRINITY.
Hope that helped at least some.
Best regards,
Janne
Ps. Don't forget to check
http://www.korg.com/triton.htm if you haven't done so already.