Steve,
This is just my opinion, take it for what it is.
I made my own edge trim from leftovers from my skin, however, my skin was made out of some heavy al and I had access to a metal break. Was sort of a pain, but I had no money for trim at the time.
If I had it to do all over again I would buy the edge trim sold at camper repair places. The kind of trim that has an insert to cover the screws. It's soft enough to bend right from the get go and you don't have to anneal it, just use it for what it is. If you have to mail order it, shipping is killer expensive. I'm not sure where you live, but if I wanted to travel a little ( I live in the state that is the camper capital of the world ) you can buy it in 16' pieces. The trim isn't that expensive, gettin' to ya is.
Expansion and contraction has me concerned on vinyle trim. I really, really considered FRP on my build as well.
Anyway, take it for what it's worth.