You need a good 28 inches of vacuum to degass that stuff, the shop vac has no hope of getting anywhere close to that. There are venturi devices to turn your air compressor into a vacuum pump but you can buy a brand new robinair type 6cfm vacuum pump for about $230 so you may want to compare the price of each. Buy a pressure tank aka paint pot aka pressure pot, dont go screwing around trying to fashion one on the cheap and risk serious injury to shop and/or yourself. Small paint pots can be had at Sears for next to nothing. I bought a 5 gallon on ebay for $199.
Note that liquid silicone (they are really stretching calling it a liquid) will expand 400% during degassing so be sure to use the appropriate sized container.
Here's a pic of my pressure/vacuum tank next to my vacuum forming table, I use it to degass liquid silicone among other things, you can also pressure form with it.