I wondered the same thing, and tried several approaches in order to save some cash. One thing I learned, rigged vacuums tend to suck (or not suck, depending on how you interpreted that), you have almost no control beyond the On/Off switch, and when they inevitably fail, it'll be at the worst possible moment. IMO, chemistry can be dangerous enough without the added risk of using a vacuum held together by duct tape and hot glue.
Since a vacuum is such a tricky thing to create and maintain, I eventually decided having consistency, safety and control was worth spending a little cash. Plus, being able to skip all that construction and endless leak patching was a nice bonus.
There's actually some really good hand vacuum pumps out there. The one I've seen the best reviews for is actually for automotive use. It's the Mityvac 04000, although the company has quite a few models. Some are plastic, some metal. Higher end ones can create a good vacuum as well as pressurize. Best thing about them, they're pretty cheap. The Mityvac Silverline Automotive 04000 seems to be pretty popular. I've seen these for sale from $29-49 USD. The lower end ones that won't keep a vacuum (unless you keep pumping) go for around $10 to $19. Here's some specs on the 04000.
Weight: 3.3 lbs (1.5 kg)
Size: 12" x 8" x 3 3/8" (305 mm x 205 mm x 85 mm)
Max. Vacuum: 25 in. Hg (85 kPa)
Max. Pressure: 25 psi (172 kPa)
Displacement: 1 cu. in. (16 cc) per stroke
Gauge Indication: 0 - 30 in. Hg (0 - 100 kPa)
Link:
http://www.mityvac.com/pages/products_hvpk.asp.
The hand pumps I've seen from labware suppliers look almost identical to this, except they're usually plastic, can't pressurize or hold the vacuum, and cost at least twice as much as the Mityvac pumps. I put one of these at the top of my birthday list. 17 days left. I hope I get it
Hope that helps a little. If not, good luck on the shop-vac solution. That'd definitely be fun to try.
-Kel