Hi there, i've been thinking along the same vibes and have been exploring my universities' engineering department (they have a rapid proto machine that would be awesome to use) Its rare that any arty types go upto main campus, let alone to the engineering block :p
Anyhoo they've got me trying to use pro-engineer - a modelling program that may do all the fancy stress testing simulation but the modelling and graphical front end looks like a slug's arse. After talking with one of the lecturers i gleaned that most places need your 3d model converted to an STL file that the CAM machines can then read and bosh ping.
The only problem with rapid proto like this is that usually it costs a bomb for a very small final thing. I have been looking at an opensource project called REPRAP - a self replicating rapid prototyping machine with free plans and code. looks a lil wobbly atm but it might eventually make clean shapes.
If you're looking for moulding and casting techniques i'd recomend heading over to instructables.com - there are some excellent videos on loads of different types of casting.
I'm glad to see more papertoy makers so interested in vinyl.
by the way, you mentioned how they spin it in a mold...that's called "roto-casting"...and peoples don't think you can't make them yourself...YOU CAN. ;)
it takes work, you need to become familiar with the process as the same as designing, it's only another medium...so, if your not used to it, you need to become accustom to it.
there are many kits out there that deal with resin/pvc/abc..for rapid-prototyping...all different compositions....you experiment enough and find your own comfortability in your liking.
ps: there are plenty of companies that will give you a quote...search for vinyl toy manufacturer....there are thousands...and if you find one, ask for samples of there work...just because there a company doesn't mean their work is good...and no, you don't have to be a famous artist to get a quote....it's called a business....so if your going to get into vinyl toys and you aren't an artist with a name....go out there and make a name for yourself, or get together with about 9 heads that love it as much as you do, and start your own company...YES!...you can do it. 8-)
although i love the idea of this....im not sure how that would actually work, in that you would have to use a hell of a lot of mold release but also the heat would also degrade your original mould quite severly.
to be honest, if you wanted to you could actually look into using different mediums other than vinyl...as lovely as it is. you could do it yourself if you used resin or a number of other materials. if you are looking into doing this make sure you make your mold from silicone as its a lot stronger and more resistant to degrading after multiple uses than latex.