there’s a chance that, in the msfs 2024 version of the vision jet, VNAV just flat out doesn’t work. it wouldn’t be a first time for many features, and it won’t be the last.
however, I did find a post from someone flying the msfs 2020 version of the vision jet (which is payware and made by a different developer–and also apparently much better than the 2024 version). he was unable to get top of descent to appear until he had activated an approach, whereupon the path system was able to calculate and display top of descent.
good luck.