Hi,
I have had the bluetooth connection fail to connect to my iphone 5s.
To solve the problem, I did the following
1. Click on the right button and swipe left to access 'Settings'
2. In Settings, swipe left twice to access 'Bluetooth'.
3. In Bluetooth, swipe left twice to access 'Pair Mobile Device'.
4. Open your Iphone Bluetooth and then open Gamin Connect and go to 'Devices'.
5. Click on 'Pair Mobile Device'. You should then be asked for a PIN. Enter the PIN.
The device should now be paired.
If your Vivoactive says "Paired" (as mine did) and won't let you pair with your phone, go to "Restore Defaults" in the watch Settings and reset. Then try and pair devices again.
Since I managed to pair the phone and the watch, the Bluetooth has functioned.