I'd think it would help alot when your approaching any sort of intersection in the dark. The car may not be pointing at you at that moment but it soon may be in your path, or you in theirs. Hopefully they'll at least look to their left before rolling through the intersection. And with the LED vest and lights it might increase your chances of being seen.
As far as vehicles coming up from behind it may help them see you a little sooner but headlights and that reflective stuff are good enough that I couldn't see it making that much of a difference there although if you have winding roads or hills it could.