The VMG coincides with the SOG, when the vessel is able to go directly to the point of destination with the appropriate route.
For example, in the case of a sailboat that fixes the destination point to the windward mark, this cannot be approached directly (but only by sailing), in this case the VMG will show the actual approach speed to the windward mark that is lower than the SOG speed (the wider the windward angle, the greater the difference).