Our first trip out west on the Valk, Sweet Tooth started telling me she wasn't feeling too good somewhere between Belle Fouche and Hullett on the way to Devil's Tower. It was over 100 degrees and after the fact we both realized she was in the early stages of heat exhaustion. She spent a long, long time in the visitors' center and ladies' room while I wandered around from one patch of shade to another and stared up at that big rock.
The next year, she maybe went a little overboard getting ready for the heat. We both ended up with Joe Rocket Sahara vests and she had a Camelbak so she could keep sipping water all the time. It was hot again across N. Dakota and Montana right up until climbing to cross into Idaho and got hot again as soon as we came down again until we were almost to Snoqualmie Pass.

My vest is easy to see in the pic and the hose and mouthpiece for the Camelback is curled up high on her shoulder on the side closest to me.
We weren't miserable or sick from the heat, but it wasn't like air conditioning, either. Like was pointed out, the vests weigh a ton when they've soaked up water (yup, these're the kind with the "crystals"). It felt like it didn't take very long before they'd get dry enough to not be helping very much, but that might be a relative thing comparing the cooling effect from when the vest has just been put on with water running off it against when it's not running wet or maybe due to the fast drying from wearing mesh over them. Sweet Tooth was NOT thrilled by the way all her white t-shirts and the bras underneath got dyed blue by the color bleeding out of the vest lining. And it didn't quit doing that the whole trip, so I heard about it the whole trip. Plus later whenever she did laundry and found some of her "blued" things in the load. And whenever she remembers it to this day.

Only other thing that comes to mind is that the vests went in the trailer while we ran around between the Cascades and the coast and they weren't completely dry days later when we pulled them back out to wear again when we hit central/eastern Oregon.
...and I haven't bothered to pull mine on since and it's pretty much 3 years later.