It's an interesting question.
As you know doubt appreciate the VSI merely responds to the rate of change of pressure in the static line ie what it is now compared to what was a few seconds ago.
The static line is shared with the altimeters so they must be 'calibrated' to hold an ISA model as the altitude rises. The pressure gradient is far from linear. The pressure drops to half in the first 10000 feet.
However the altimeter knows how high it is whereas the VSI just knows whether there is a pressure difference or not across its bleed valve.
My guess is it's incorrect at altitude - under reading.
The reason I make this wild guess is you are generally more bothered about whether it is going up or down rather than by how much, except near the ground where approaches and departures are concerned with real altitude profiles.