I put the VHR into broadcasting mode - pressing the side button, scrolling to the settings icon then sliding to Heart Rate, pressing it, sliding over to broadcasting and then pressing the check icon - paired with my Garmin 920XT in the same way you'd do any other HRM (I now have three devices paired). I started running, thinking I was good to go, but it seemed that I must have missed a step or the initial pair didn't work - when I went into indoor running and started the activity on the 920XT, it wasn't showing my HR a couple of minutes into the run. You can see this on the red chart below as its flatlining around 60bpm.
I stopped the treadmill, re-paired, and started the run again (you can see where it happened on the upper (blue) chart.
The monitor in the VHR broadcast to the 920XT worked like a charm -- it showed approximately 130bpm during my 9-10 min/mi pace on the treadmill, jumped up to around 155-160 during two 7:15 min/mi interval sessions. I slowed down towards the end to a walk until I got my hr down below 100 - which usually takes about 2 minutes when running outside using the HRM Run strap, same with using the VHR.
The strap was not secured "very tight" -- tight enough not to slide loosely, but not so tight that it was uncomfortable for me (princess and the pea).
One side note:
When I ran on the treadmill, those minutes were reflected in "intensity minutes" metric, even though I didn't "create" an activity on the VHR, just on the 920XT.