I've got that to work (volume control of VP with ATV remote).
I adjusted the GLM software volume to 0db for the test, as I wasn't getting useable (big enough) changes in volume for each button press and also level 1 of hundred wasn't very quite and then 0 of course was silent.
Google wisdom suggests the best place to control digital volume is in GLM, so I've reverted to the volume knob as it's more adjustable, quicker and the family are used to it.
The exercise has meant that I've stumbled on settings across, my new TV, apple and VP, that re enabled mute, volume control (which I'll ignore) and TV on and off, which the addition of a new samsung TV has screwed up.
Back to just enjoying this endgame system