Hey dude.
Same here mate....
I get the basic shape out of clay and vac form it. Then if it's rubbish, change my clay mould until it's CLOSE. You can use the clay again. Or for small things I use air hardening clay ad then just use them.
Change your mould and pull it in plastic again.
Take your formed plastic off, then line the negative plastic shape with liquid vaseline and pour plaster into it.
Depending on what your doing you can use household brown plaster...?Most of the time I use Krystical Alpha K plaster. Very high detail and very high density. This makes it easy to carve/file as it won't fracture.
Wood carving chisels work great on plaster if used correctly. Keep em clean. That's how Vader was sculped too 
Then pull away!
Make a note of undercuts and ease of taking the sheet off the mould and where you need fine detail use a piano wire in a drill chuck (bend it back on itself a few times) and score back and flatten the end of the wire and drill through to the bottom of your mould. Or just a 1mm drill bit. Usually very short though. Do these in corners etc to help the suction pull into the detail.
Never place high edges close to the edge of your forming area and never place high edges close to one another if you plan on forming several parts in one go. Otherwise they web. You can cheat and ush down with a cloth whilst the plastic is still molten but I'd rather avoid it and play safe.
Me and a friend made a kids TK suit in a day! So give us a PM if you need any info.
It's not hard
Good luck
