Summertime here is more muggy than not, with heat index at the lowest 95, if we're lucky. Usually it's over 100. :(
I do a bit of flatwork with jumping. No plans for eventing, but I am planning on jumping more within the next year and getting into showing. We do have jumps set up on harder ground though, in an area with trees, so we do have the standard jumps as well as a variety of "natural" jumps set up, including fallen trees, brush box, etc. An arena is coming soon! (So happy about that, and the slightly softer ground..).
I'm usually an extremely careful rider, and last week was the first time I'd ever been thrown off, and it was twice in one ride :( I have really good balance and don't push a horse too hard if I know they will act up if I do.
I'd definitely wear a vest on a few of the horses I ride, no matter what the weather, simply because they are less predictable, though my horse I trust enough to not need a vest when I ride him (I do always wear my helmet though).