It will give an answer through trial and error as you sail a downwind leg if you put in a waypoint where you're headed, then try different headings and speeds until you maximize the vmg. It's essentially telling you if your actual speed exceeds the distance penalties you've worked out through trig (5.1, 5.3, 5.8, etc.). But if you mean will it statically predict a vmg for a given boat or wind condition, no. That's where the polars come in, and as noted they are highly individualized. Also, I think most racing skippers will tell you that they "know" they can beat or can't quite make the polar numbers in certain areas of the graph: they remain estimates, especially when a boat model is new and there is little experience to feed in. Finally, sea state can have a huge effect on the polar performance. There is a lot of difference between sailing in flat water vs reaching hard into a head sea (which may not be the same on each tack) and vs. broad reaching on waves sufficient to provide a surfing boost. For fun, you could throw in the difference between salt and fresh waveforms. Our relatively light CSs may take a huge hit on upwind performance in a chop, but could pick up a huge lift from a fairly modest following sea, especially if the skipper and crew can "ooch" it right. The onboard vmg reading will give you that, where a polar can only give a predicted estimate.
All obvious, maybe.