I think Santos will definately be the next president. I just don't see the show having a GOP president simply because then they'd (somewhat) alienate themselves from the (somewhat) established fan-base for the show. That and it just seems like Jimmy Smits would be more likely to sign on for a long-term run with the show.
I have to admit though, Vinnick has been portrayed as an incredible guy. After his acceptance speech, I can see him becoming the next President, though I hope that doesn't happen. My guess is it will be Santos, and we'll have Josh that way. Donna, I think, will easily (and eagerly, if the last few episodes have been a hint) move to the Santos camp...Santos certainly deserves to win more than Bingo Bob.
As an aside, the next episode is the season finale, when we find out who the Democratic nominee will be. Do we know if they plan to use the gap between this season and next season to just skip over the campaigning and election, or will most of next season be the heading up to that (which I think would be a bad idea, since we've gotten enough of that atmosphere this season)?