Shop vacs standard measure is inches of Water Gage, a far cry inches HG. Furthermore, most vacuum hold down systems (this includes the big industrial CNC's) rarely measure vac pressure at or near the table. A simple test of holding power is to mount a vacuum gage (HG) onto a gasketed phenolic panel (perhaps 6" x 6") and place that where you would be placing your work piece with vacuum on. Divide the value you get on the gage by 2 and THAT is what you're getting in the way of PSI at your work piece. That will quickly point out many flaws in a vac hold down system.
Another thing to consider (wizard mentions this) is side slippage. Cutting forces are generally parallel to the XY plane. If you are cutting melamine, chances are greater that it will slip as opposed to a veneered piece of MDF. I've used peel and stick abrasive paper on my work piece fixturing to combat this. Obviously, the abrasive wouldn't work well on a conventional spoil board.