I've got a Vivoactive HR and a Garmin 1000. I paired a Wahoo HRM to the 1000 and did several rides to compare the HR stats between the 2. I found that while the Vivoactive had some large spikes in HR the average HR between the two was spot on.

FYI - I use the Vivoactive HR to track all of my activities but, I still use the 1000 on my bike because I just want to look down for my ride status, I don't want to be distracted by looking away from the road.

Also, I have not had any problem pairing my ant + sensors with the Vivoactive. I do have the new cadence sensor and need to make sure and pedal my bike to wake up the sensor before I start a ride.