If you're referring to pressing buttons on an existing joystick on command, that is not supported by the Windows joystick system (this is not VoiceAttack-specific).
Rather than configuring TrackIR to use joystick buttons, have it use keyboard keys instead.
Then, create a VoiceAttack command that can be triggered using a spoken phrase, as well as the joystick button you were using previously, and have that command press the relevant keyboard key for you.