I managed to get all the way to the end. I had all kinds of very powerful guns, modded to the maximum the game would allow. Even so, I must have tried to save Kent 25 times in succession using every tactic I could think of. VATS or not, no matter which gun I used, Sinjin never got killed by the first shot (even after intimidating his men) and VATS actually paused to allow him to move with me frozen to kill Kent. Very sad and very dissapointing.
Maybe this is one of those "I'm not a twitch gamer" things, but it was just impossible for me. I even turned the game down to very easy and there was no effect on the outcome. I'd almost rather load an old save but at least this way I can chuck the stupid costume out of my inventory.
I'm really ashamed of Bethesda. I even tried stealth boys but the game didn't seem to notice I was invisible, the same events just rolled.