Tracy,
If you don't break through the onion skin, you won't lose suction. The idea is to let vacuum hold down your parts while cutting aggressively (tool pushing back against material with high force) - without breaking through the skin. Then when all of the parts are cut, say .7" deep in .75" thick material, come back and make the tool go .75" deep, cutting all the way through your parts - now breaking through the skin and inevitably letting the vacuum leak through the kerf. Since the tool is only cutting .05" off of the part on the last pass, there is very little force being exerted, and therefore, the parts shouldn't move even with low vac showing on the guage.
There are other things that come into play depending on what you are cutting, how grabby the material is, how high your chipload is and the geometry of the tool being used to cut the parts. If your parts are still moving with good vacuum numbers and onion-skinning, you may want to look into a compression spiral, or even straight flute tool. In more extreme cases, a downcut can be used, at the expense of blowing out the bottom edge and packing the kerf with chips (straight will pack them also - sometimes to your advantage, such as with plastics.)
Does this make sense?
-B