One major problem with a manual pump is the time required to create a full vacuum. A shop-vac will create much more than sufficient suction for molding cases. In my case, vacuuming in a Canadian Winter, I had little over 20 seconds to take the heated plastic out of the oven, out the door, gently place it over the mold (vertically downward over the weatherproofing seal, being careful not to stretch it) and turn the shop-vac on before it solidified and became unmoldable.
I suppose this is for a warmer climate?
Not sure about the climate issue, unless Canadian plastic adapts its thermoforming temperature to the climate.
How thick was your plastic, and what was the shape like? Was there fine detail or deep draw?
In my experience, high vacuum gives you about 2x or so improvement in resolution, approaching the thickness of the plastic, so you get about 4x as much "information" showing. (Actually better if you form into a female mold, so that the mold side is what shows.)
It sounds like you must have been forming reasonably thick plastic, or you wouldn't have had 20 seconds before it cooled too much.
The time to draw down the tank isn't a big deal if you're doing one-off case work, and you don't use too big a tank. If you're doing production work, you want a reasonably serious vacuum pump to draw down your tank in about the same amount of time it takes to heat the next piece of plastic.
(BTW, Doug Walsh has a good deal on surplus 4.5 CFM vacuum pumps at www.build-stuff.com
right now---less than $100 shipped---so even a high-end setup isn't horribly expensive. The plumbing and a big tank cost less than the pump.)