Just another thought about vests. I ride across a two lane huge suspension bridge most of the time when I go for a ride. I usually come back over it after dark. There is room to ride a bike safely next to a car, but no way to allow extra room if a car is too close. I was riding across one night and a cyclist was going the other way. As we were about side by side I looked over at him. A car was coming and the lights were on him. He had no lights at all. He had a safety vest, bike reflectors, reflective ankle and arm straps and possibly some on his helmet too. I remember thinking WOW that guy is bright! I think I have seen him during the day. He uses his bike as transportation, I think he has no car. He has no worries about buying batteries either. The gear he has is not expensive and will almost never wear out. It was incredibly good, I was amazed. I wish I had a picture to post, it might make a few new vest wearers. Also he was clearly a rider on a bicycle too, no confusion about what he was.

If you have not seen the newest brightest LED taillights you may be surprised. I have a couple that are MUCH brighter than car or truck taillights. They are much more visible 1/2 mile away than a car. They stand out in traffic among the car taillights from 1/2 mile away. They are very bright during the day too. They help drivers give me more room during the day than anything reflective that I have tried yet.
But only the newest and the best are that good, there are many LED taillights that I consider almost useless. Unfortunately most riders probably have older weaker lights. Also the angle is important, the light has to be horizontal to be visible, and most of the older style lights are not highly visible from the side.

My vote is not vest or light. It's vest and light. They each have a benefit.