You have to set a target altitude that is lower than your current altitude by at least about 100ft or so. I usually set the lowest altitude I want the VNV to take me to, for example, the FAF altitude when doing an approach. Also, with the current version of the G1000NXi, your next waypoint has to be at least the first waypoint for a procedure such as the IAF for an approach. My understanding is that currently VNV has not been implemented to take you from one cruise altitude down to another cruise altitude. And of course, VNV only works for descents, not climbs.