I think you are right - not so widespread but simple and useable nonetheless - you know how many Horspower you use to fly at different speeds - right? You know how long you can fly for using only the energy from airspeed from 100kts to 90kts? It doesn't take too long to work out the figures and you might like it.
The unit of VSI is in 100fpm
A HorsePower is 330lbs raised at 100fpm
So every 330lbs you weigh is 1Hp for 1unit on the VSI
If you weigh 3300lbs then its 10Hp per unit of the VSI ... not essential but interesting.
'Soviet units' km, m, m/s don't work so well for flying.