I don't believe we are speaking of quite the same thing. I am making reference to a Baro VNAV path that is locked in the nav database as part of the instrument approach procedure, not a path constructed by the flight crew.
Actually, that was what i was talking about. Purely database derived approaches, it is not VNAV anymore with boeing as that is old fashioned and put to rest in favor of IAN which behind the scenes is the same thing.
Baro VNAV still relies on baro settings and only can be as precise as those. And baro settings are inherently inprecise, unlike purely geometrical approaches like GLS, MLS or ILS. One hPa equals roughly 30ft in height difference at sea level and that is something you will have quite often during rapidly changing conditions. Crossing the threshold at 80 ft (50 plus 30 for one hPa) will increase your landing distance with picture perfect flying by around 200m in a 737, which on a marginal runway could be outside limits (mandatory GA). That is why boeing advised us that the database derived glidepath might not be a good idea to follow below MDA/DA and that segment should be flown visually only, GPWS "glideslope" alerts can then be ignored as they are very common.
Don't get me wrong. Baro VNAV is a nice tool, but one has to be aware of its limits.