DuracellEnergizer said:But in one timeline Doc gets shot and dies while in another he wears a bulletproof vest and survives ...
We don't see him die at the start of BTTF, we only see him shot. The vest reveal takes place after Marty has gone back to 1955 and after he's returned from 1955 LOL. It's possible that the Doc had the vest all along and the whole thing is just a closed time loop.