My VO2 Max on my vivoactive 4S is not updating.

It won't show you a new VO2 max unless you set a new VO2 max.  So if your VO2 is 45, it won't set or show a new VO2 Max until you improve to 46 or fall back to 44.  Also it will only measure VO2 max in certain circumstances (must exceed 70% of max heart rate during a GPS Run / Walk / Cycle activity for a given time, which is 10 minutes for running and walking, 20 for cycling.).  Plus, there is a hierarchy, where a walking GPS activity will not set a new VO2 max over a running GPS activity.  Read the below link for further.  It's a great feature, just takes a little getting used to.  For instance, you can run a mile every day, and go from a 9 minute mile down to a 6 minute mile, and your VO2 will NEVER increase according to the device, because you failed to hit the 10 minute parameter.  As long as you know that, slow down and run for 10 minutes ( or a little more to make sure you are in the 70% of max heart rate zone) and you will get a new VO2 if it has changed. 

https://support.garmin.com/en-US/?faq=lWqSVlq3w76z5WoihLy5f8#e

Hope that helps!

Thanks so much - that is very helpful.

Do you know how long it takes for an improvement in vo2? I’ve been training hard for around two months with no improvement :(

That doesn’t sound right.  Mine has improved with just walking and a tiny bit of running.  I would have thought if you were trying hard you should definitely have seen an improvement.   Did you see the answer to my question above?   It does have parameters.