Interesting...I'd not heard that.
But wouldn't the the ANSI II standard require reflectivity even when wet? After all, the standard is designed for workers standing next to 55 mph roads, and they're sometimes out there when it's raining.
I haven't looked up the ANSI test standards to see if they include a wet test. I guess an easy way to find out, is to get part of the vest wet and then do a quick reality check with a flashlight.