(1) (Value Added Dealer) See VAR.
(2) (Voice Activated Dialing) Dialing a phone by speaking the names or numbers. VAD is a common capability on smartphones.
(3) (Voice Activity Detection) An algorithm that determines when an audio signal is a human's voice. VAD is used in voice over IP (VoIP) and speech recognition systems in order to not waste computer processing cycles if the sound is not human speech.