My seats have decent vinly but the bottoms of the backs are rotten where they meet the base. I plan to pull off the vinyl and rebuild the frames then hopefully pull the vinyl back over them.

Question: How do I make the curved back piece? Anyone have any suggestions or good ways curve the back around the bottom? Thanks.

Good luck removing the vinyl. You usually damage some of it in removal.
Buy a good tack puller at a fabric or upholstry shop to remove the staples and try not to rip anything. The vinyl may be attached internally in the seat too.
1/4" plywood can flex pretty good. If it wont make a sharp enough curve, you can cut half way through the plywood every inch using a table saw then curve it around an arched piece. Glue and fasten it tyo the arched piece.

3 ways with wood..
1. force it
2. steam it
3. lots of almost penetrating slits with a table saw

done it a few times, carefully take it apart, just rebuild just like they did it..pretty strait forward when you take apart, once wood done, paint it with a couple of coats of good ext paint...they probably left the wood bare...

your vinly will probably come apart plan on the extra cost of replacing that specially if they glued it

A heavy coat of thined epoxy to soak in then a couple regular coats and never worry about the wood again.

on my bolster seats the outside is round in the back of the seat. once i pulled the vinyl and foam off it was 3 small pieces put together. good luck on removing the foam not the vinyl.

cut at 45degree angle to the outside and sand the shape edges

I did the same thing and got tired of the whole wood thing
Went to the local plastics shop bought a couple of sheets of PVC
used the wood as templates and then just screwed and glued the frames together........ just my 2 cents

when i redo my seats again im going to the pvc. the price is only a couple of dollars more a sheet then marine plywood, and is half the weight plus it will never rot.