In the first trimester, the embryo is well protected. I had a horse trip and fall on me when I was about 6-7 weeks pregnant and the doctor was pretty relaxed about the whole thing (my son was fine).
The biggest risk when riding while pregnant is separating the placenta from the uterus, which most likely would occur from a fall. I do know someone who miscarried at 5 months after a fall.
Would a vest help in that situation? Not really.
I did ride during both my pregnancies, but in a very limited capacity. My first pregnancy, after the horse fell, I rode only my own horse, who was a real steady Eddie. I never rode outside the ring, and while I would get on and walk, my need to pee was so extreme that it wasn’t really worth it. In my third trimester, I mostly did groundwork.
During my second pregnancy, I stopped riding at 5 months because I felt uncomfortable and unbalanced. I did groundwork and had someone else keep my horse schooled. Your ligaments loosen considerably while pregnant which changes how it feels to be on a horse.