If Vince is the starter, he should be a decent fantasy play in leagues that give 6 points for rushing TD's and 1 point per 10 yards rushing. As a full time starter, I'd expect him to average 40-60 yards rushing and maybe .5 rushing TD's per game. Throw in some passing yards and the occasional passing TD, and he could easily score you 12-18 points per week (despite likely INT's and fumbles) with potential for higher. One good thing is that even in blowout losses, I'd still expect Vince to play the whole game and get some garbage time stats. They will be looking to get him experience regardless of the score.
I would not be shocked to see Vince start against Dallas in week 4, though I would also not be shocked to see him not get a start until week 8 vs. Houston. Personally, I think it will be sooner than that though. One or two more ugly losses, and the Titans will have nothing to lose by putting Vince in, and the fans in Nashville are already wanting to see him now that Volek has been ostracized and Collins stinks.