Currently it is possible to effectively emulate that functionality using commands.
E.G. configure a command to trigger when V is pressed, I.E. with the "Shortcut is invoked only when all keys are released" option disabled (unchecked), containing the "Make VoiceAttack Stop Listening" action,
and configure another command to trigger when V is released, I.E. with the "Shortcut is invoked only when all keys are released" option enabled (checked), containing the "Make VoiceAttack Start Listening" action.