I am not sure that one vest isn't enough, although I do own several.

If I were gonna have just one, it would be a screaming yellow wind vest like the PI whisper or vagabound II jacket (in vest mode), and just put warmer layers under it as needed.

In fact, the PI vagabound II jacket is my one piece I use the most, either in vest or jacket mode. If its very cold, I move to a warm jacket like the PI gavia or shebeest windpro jackets. Again, the PI gavia is available in screaming yellow, so if you want 2 pieces, I'd suggest getting both the PI vagabound II and PI gavia, both in screaming yellow, and you'll be covered for a very wide range of temps. That doesn't mean you shouldn't be sure your bike is well lit, etc., but screaming yellow really works.

I have an older PI whisper vest that is a size too big, so I can also layer that over the shebeest windpro jacket if I need more visibility. So, it also depends on whatever other pieces you already own and of course the temp range you ride in. I picked up my gavia and windpro jackets on sale, so didn't get them in screaming yellow. But if I were to do it over I'd buy fewer pieces even if not on sale, but in screaming yellow.