I just stopped riding when it's above 90 degrees.
But before that I tried lots of thing, I was unimpressed with cooling vests. The stuff that they absorb water with leaks out when the jackets are wet,
They don't flow enough moisture slow enough to really do a good job, they keep the wetness on the surface of the vest but block much of the cooling effect from your skin, where you want it.
I've had better luck with a thicker cotton shirt (I used the striped locomotive ones made by Ben Davis)
I get that wet and wear it under a perf jacket, the windflow cooled me off whenever I was moving.
But in hot weather it would only work for about 10 min then the shirt needed to be rewetted.
I thought about making a vest from some microfiber cloth, but then decided that I'll just drive when it's stupid hot out.
On vacation for a spell