I'm sold on seat harnesses.
Maybe wear the Nitrous? Anyone tried a Nitrous with an impact vest?
I've been wearing Nitrous shorts with a Dakine Slam Vest for a little over a year. Never had any issues with the vest riding up. It fits fairly snug, which likely helps with keeping it from riding up. The only issue I've ever had with this combo is the zipper at the top of the vest sometimes starts to unzip. No big deal though.