in my opinion a vest is not only a fashion accesorry, but also a helping tool to get all the stuff you need - with you - on the firing line-
to gain more points? - I dont know, maybe one or 2, maybe a 50% if you believe in it- after all it is all in your head........
One thing that is very helpfull is the leather (or non slipery material) recoil pad, it helps in 22s by preventing slips, and in HP it gives you an extra cusion + non slips. Because our rules say that there has to be 4" room, they take any stability advantage out of the vest.
I shoot with the vest, and I can not imagine to do it without it, but for me the biggest advantage is the recoil pad, and if I could put that recoil pad on my T-shirt then I think i would be shooting the same.