This is a known issue:
User is unable to change VVM playback volume via earpiece, speaker, or wired headset using hard volume key.
VVM uses VOICE_CALL_steam type to play the sound.
Although VOICE_CALL_steam type is controlled only by call mode or communication mode,
VVM set normal mode which block the volume control.
T-mobile confirmed that it is a limitation of the current VVM SW. According to TMO, a new SW will be available through Google Play update.