I have a Vivoactive HR and a Scosche Rhythm external armband HRM and a Polar H10 external chest strap HRM. I find the on board wrist HRM to be totally unreliable even just for monitoring daily non workout activity. So I thought I would just use my Scosche armband for daily monitoring and the chest strap for workouts. First found out that both externals would not stay connected to the watch unless an activity is being Recorded. Second found that both devices would frequently but not always disconnect from the watch during workouts and HR data would revert to the wrist HRM which always gives very poor readings at 30-60% lower than actual correct readings. Looking around I see that I am not alone and many people have reported similar issues.
I seem to have come up with a solution. First Go into the Sensors settings connect your external. Then go to the Wrist Sensor settings, turn on both Broadcast In Activity and Broadcast. While in Broadcast mode your external will remain connected full time and the wrist monitor shuts off. While in this mode you will have access only to a single large display of current HR and RHR. This suits me fine since I have been unable to find a watch face that has a large easy to see at a glance display of HR. Broadcast In Activity will keep the device connected while recording an activity.
Hope this helps people out there. Worked for me and my devices anyway.