I always have the little rain cover I made for the top of my Richardson that takes up about as much room as a folded sheet of paper. I also carry one of those throwaway ponchos.
The only time I carry a proper rain jacket is if I expect it to rain, a LOT and my rain jacket is a flyweight nylon job that folds up very small. I hate wearing jackets anytime for any reason so it takes a real lot of rain to get me to break mine out. Both the cheapo throwaway poncho and the rain jacket fit just fine over my Richardson box, that's why I carry them.
A vest to me is a place to put stuff I don't need enough to bother carrying, but I did because I had the room for it. That's why ditched the vest in the first place and went with a chest box.