I fully agree, one should not be needing/using any Vnav guidance then. However while in IMC being radar vectored the distance to go is another tool in your arsenal of situational awareness and can't hurt.
The distance is calculated
directly to the next active waypoint and not to a ficticious point on the centreline abeam the aircraft.
The displayed dist to the destination ( prog page 1 ) is then the least/shortest distance to run and you shouldn't be caught out too high.