The reason I'm looking at it is that when the watch was launched in 2015/2016, Garmin Connect seemed to report VO2Max, and it seemed to move around / improve in line with fitness quite well (even if it is an exaggerated number). But VO2Max calcs are supposed to only be done by higher end watches, so my theory was that this was being calculated on Garmin's servers as they ran your pace / HR / weight data through the FirstBeat Analytics software that they own. But then in April 2016 the VO2Max data stopped being updated by them and after asking Garmin about this they said it was a bug and should never have been displayed for Vivoactive users as that feature wasn't included with that watch.
But if the value is still present in the file somewhere, it is either being calcuated by the watch but being suppressed from being displayed, or as you say it is being added to the FIT file by Garmin's servers, which implies it is still calculating it. I need to wait for my garmin account to sync to Runalyze for the last 30 days now that i have authorised syncing to see whether the value moves around, or if it is just a static one based on where my VO2Max was last calculated back in 2016.
The number does seem a bit high - 58 or so, but it's not a million miles away from where I might expect it to be (46 year old, 18:14 5k time)
The vivoactive is the only Garmin watch I've ever owned, so the data can only be coming from my Vivoactive!
Thanks for such a quick response!!