I've vac-formed Vivak and Polycarb. Big difference. Vivak forms nicer, gets nice and saggy. Polycarb not as saggy, but still works. Polycarb is much, much stronger. I tried to form 1/4 polycarb one time, didn't work very well, maybe the heat wasn't high enough or I didn't let it soak at that temp long enough.
If you could do 1/8" polycarb, I bet that would make a wicked strong fairing. I've always wondered why they don't do something to beef up the edge a bit, but there's probably a good reason for not doing it.
Katou