I work in a commercial cabinet shop with a large cnc router with a vacuum pod system. We have a VERY large pump and it will draw through MDF a little, but not enough to hold parts still. Most large commercial routers that draw through MDF spoilboards use even larger pumps. I'm not sure exactly how they are rated, but these pumps are larger than the cnc's we're talking about here and VERY expensive. I think I have a better solution. Last year I built a vaccum veneer press with a compressed air powered vacuum generator for under $100. This will draw much more vacuum than a shop vac. Go to
http://www.joewoodworker.com for the article on how to build it. Then get a copy of ShopNotes issue 40, which has plans for various hold down jigs and a vacuum table, which should be a good starting point for a cnc vacuum hold down table.
http://www.shopnotes.com/
This is what I plan to use on the router I'm getting ready to start building.
Gerry