Vertical Acceleration (VA) is a speed up of the focus servo in a vertical direction in an effort to maintain focus. Consequences of VA are similar to RA, the head loses focus and the drive has to slow down. Any error in the replication process that causes a "bump" on the disc (example a dent) can cause VA on a disc.
Problems caused by the stamper are easily located by high RA and VA measurements at the same point on different discs. The consumer sees both RA and VA most pronounced in games and multimedia ROMs as a slow down or drag in the program.