Shouldn't be mandatory, but should be encouraged. Not neccesarily vests, but when you see some of the stealthed up riders out there you have to wonder what goes through their heads, it's as though they want to make it harder to be seen. No reflectors, no reflectives, black clothes all over, and a tiny little rear light from Tesco. Mental, frankly.
I don't generally wear a vest but my bike's lit up like christmas and has every reflector I've ever owned attached to it. My bag's got retroreflectives all over and at night a light or two attached, my helmet's got reflectives, most of my riding kit's either bright or has reflectives on too. Sure, some idiot driver could still hit me but at least I'm not giving them any excuses.
One last thing... Lots of reflectives out there are actually terrible. proper quality retroreflectives are excellent, and in a car headlight are more visible than even bright lights but the excuses for hi-viz that sometimes get sold (often specifically for motorbikes and cyclists) with pitiful yellow reflectives that barely bounce any light, and bounce it off in the wrong direction... Not useless, but if you're going to do it, do it right.