Ah yes, VCALC vs VNAV. I believe
@flyingcheesehead can probably explain it better than me.
I'm not sure what functions the newer more economical boxes have, but here's the difference.
VCALC lets you manually set a distance before the destination, an altitude above the destination, and a descent rate and it will alert you when to start your descent.
VNAV lets you enter altitudes for any point in your flight plan, including multiple points and along-track offsets. If you have any glass to go with it (G5, G3X, G500, etc) you'll actually get a glideslope indication, and if you have a Garmin autopilot it'll automatically initiate the descent if you press the VNAV button and set the bug to the lowest target altitude you've been cleared to.
With a GTN, VNAV requires an altitude source to be interfaced with the GTN. Right now I have relatively "dumb" steam gauges - My altimeter provides the baro setting to the autopilot but that's about it, so I would have to hook it up through an air data computer. As a result, VCALC is all I can do. If I added a G5, I could do VNAV.
I'm not sure whether the newer, cheaper units (355, 375) will do VNAV right now or not, and if not, whether they'll get software updates to add it in the future. The GTN didn't have VNAV until last year (v6.50 of the software) so even if the 355/375 don't do it now, they may in the future.