I used to own a Vista, before it disappeared between many large boulders. I found its altimeter to be extremely accurate once it has been on for 15 to 20 minutes. I believe Garmin described it as a GPS corrected barometric altimeter, this is how I think it worked; it would average GPS readings over a period of time and correct the barometric readings, then any small changes in barmometric pressure would change the elevation reading. Not sure exactly how all the calculations were handled but I thought it did an excellent job, often reading within a foot or two of the surveyed elevations of Colorado Mt. Summits. Of course in other areas it could be off by 20' to 30' of the Topo lines but they are only accurate to within 50' themselves so you never know what the correct reading should be.
Basically what I have been trying to say is the the Vista Barometric altimeter is not strictly barometric but a combination with GPS.
Maybe you have tried this but it is interesting to leave your GPS at one level when you first turn it on, watch the elevation slowly track one direction or the other, when it finally settles in to one elevation you know that is as good as it will get.
