The Stalker, the Rustler/Merco 40, the Ro-jett 40, and the PA 40 (both the regular and the ultra lite) are the only engines that I know of in the "40", or so, size that are rear exhaust. The PAs and Ro-Jetts are the "ultimate" for this size plane. I have not seen any of those in use on the Vector 40, yet.
Jim "Uncle Jimby" Aron has a Vector 40 with a piped PA40 "Ultralight". I flew it on New Year's Day. Quantity of power is NOT AN ISSUE! His still needs a little work on tank shims, etc. It also worked very well with a Magnum 36 on a pipe (as did 2006 Senior National Champ Paul Ferrell's).
The Vector 40 doesn't require a tremendous amount of power. Peter "Greasy Pete" Deane flew his on a Fox 40 Stunt (Large Case), and while it took a few laps to get up to speed, it flew surprisingly well. It's a very feeble 40, probably less powerful than most Fox 35's. His has a split-case muffler that has the primary effect of distributing oil over every exposed surface of the airplane, all over the launcher's feet and hands, and the flying circle. Hence the name "Greasy Pete".
I would think even vintage 35s (Fox, etc) in good shape, on 15%, would be plenty of power. The Stalker 40 RE's I have seen are less powerful than a Fox, and would be about as little as I would suggest. The upper limit is whatever you can stuff in there, and still get it to balance, and control the power. For the larger engines, it had better be pretty mellow.
If I had one, I would use a Rustler-Merco 40 Metamorph. Lots of power, very mellow to control, and light.
Brett