Remember, the force holding an object to the table is actually the atmospheric pressure. When you remove ALL the air from under an object, you get the full atmospheric pressure pushing down (not really possible, but it is about 29"hg or ~15psi, so about 15lbs of force on every square inch.) With a shop vac, you are probably getting a maximum of 3 psi. So if you want 1000lbs of pressure holding your piece down, that means the surface area of your piece must be 333 square inches (11" x 30"). With leakage, the pressure will decrease so you either have to cover up the rest of your spoil board, or get a vacuum source that has enough flow to maintain vac pressure at higher leakage.
If you are cutting whole sheets (4608 square inches), you will be getting over 12000lbs of hold down force from a shop vac as long as you leave an onionskin.