I have both a Vivoactive and fenix 3 and yes, the Vivoactive works great (even with having only "smart" recording for GPS). Distance and elevation have always been close to what others have had on the same rides and runs and match up with my edge 800. My fenix 3 way under reports my elevation gain which is a bummer for those Strava climbing challenges I've entered. Kind of defeats the purpose of having the fenix 3 if it is way off on elevation gain.
Only issue I have with the Vivoactive is that I'm seeing much less than advertised battery life (just under 7hours for me) and there is still no app for a power meter. Those are deal killers for me.
If only the fenix 3 had the GPS quality of the Vivoactive....