Jcooper,
The natural wash in a hover will cause a momentary dip in the VSI, and also in the altimeter. Remember that the static ports are usually in clean air until just about hover speed, when they become immersed in the rotor wash. Some flight manuals have the altimeter calibration curves in them, take a peek and see how speed affects the static ports of helicopters. it is not unusual to see a 30 to 40 foot change in the speed range of the helicopter. remember that about 1" of altimeter (mercury) is 1000 feet, so a 40 foot change is .040" of mercury pressure change. if 30 inches of mercury is 15 pounds per square inch (close enough in my book to 29.92 inches and 14.7 psi!), then 1 inche = 1/2 psi, or .040 inches = .02 psi. Small potatoes!
We covered the whole ground effect thing a while back here on pprune. While there was no consensus (unlike California, where there seems to be consensus.....better not go there!), some of us strongly advocated that there is no pressure bubble under the helicopter in a hover (the belief otherwise is the kind of intuition that helps us think about hover ground effect, but is actually not true aerodynamically.)