Vacuforms are relatively cheap, but the trouble is, no one wants to buy them. The perception of vacs is that they're tough to build, and you're catering to a very small market. You're also very limited with the amount of detail you can incorporate into a vac. They lend themselves to aircraft fairly well because most fuselages and wings are smooth surfaces. Ships tend to be far more angular, with tons of recesses and surface details...
Resin casting will probably cost you about the same amount of money in the end, and that's about as close to injection as you can get without spending huge amounts of money. Trouble is with resin, is that your kits or parts are only as good as your patterns. Add to that the plethora of companies that are already doing it...L'Arsenal, Corsair Armada, White Ensign...they're pretty much state of the art...
I had a resin ship company for a few years, called Accurate Image Models (AIM), and it was a real pain in the arse. Casting, pressure pots, making new molds, cutting parts from trees, packaging, drawings instructions, doing the photoetch artwork, then listening to people complain because your 578th hull has an air bubble on the side.
Best advice, build for fun, enjoy it...
If you really want to know how to make a million dollars in this industry...it's very simple...start with two million!! :-)
Jeff