I was also wondering how or what the Vo2 was exactly on the Garmin. I have a Vivoactive 4S, I have had it 3 weeks. In the Vo2 it says I’m at a 51 (excellent), top 15% for my age and gender. Fitness age is a 20. Sort of shocked me because the whole reason i have the watch is because my wife wants me to get into shape. I am a 36 year old truck driver with quite a belly, haven’t seen “exercise” in at least 8 years. I’ve felt heavy, tired and slow during those 8 years. On the other hand my wife is in shape, eats healthy and works out all the time and her Vo2 is 37 
But i have bought a bike and will purchase a Saris H3 and trainer road next week to get this body moving again!!