The VVI is acting correctly. When you pull up to climb, the VVI will show a descent due to gravity and show a climb when you push forward to descend. Once a climb or descent is established, the VVI will catch up and indicate your rate of vertical speed in accordance with power and pitch used. Think of it as if you have a glass of water setting on the dash of your car, accelerate and the water goes back, stop and the water goes forward.
Wheel