If you’re training outside, there are too many variables at play to use VAM as a training metric. I wish it weren’t the case. For example, lots of people fixate on 1,000 m/hr as some kind of magic number. However, the grade (and changes during an effort) have a huge effect.
* If you’re on a 1% grade, can you climb at 1,000 m/hr? No, you’d have to run 100 kph.
Also, the inaccuracy of the barometer in training watches and the lag in the calculation.
However! If you’re willing to train on a quality incline trainer with reliable speeds, it’s an awesome way to train. I had my biggest gains when I spent a year doing all of my intensity on a calibrated treadmill at 25%.
When I train outside, I go by heart rate. When inside, by vertical pace at 25%.