People would complain (rightly so) that characters that had an entire movie re-introducing were wasted by dying. Carol could've died, but not Spock and Kirk.
You don't have Star Trek without Spock and Kirk.
Well, sure, but fans complained that Abrams didn't do anything different enough, as well as the fact that Nick Meyer intended to kill off Spock permanently. The follow up shot on Planet Genesis of the torpedo was a studio mandated pick-up.
I think that you can kill of Kirk and Spock, if it follows from the theme of the film. Actually, I think Meyer said that at one point. The brave part of TWOK was the fact that Spock was killed and didn't come back at the end. Also, Star Trek Phase 2 was originally slated to not have Spock.
As much as I like Vin, he is no Jean-Luc. Vin is a man of few words while Jean-Luc is a verbose speaker. Maybe Richard Armitage, but not sure how he would look with his shaved head.