Vietnam set up defensively against the championship favourites, and were able to stem the tide in the opening exchanges, but launching from range Iran were four goals up at half time.
Vietnam conceded four more in the second half, even as Iran took the foot off the metal.
The only bright spot for Vietnam was Duc Hoa's goal in the 38th minute of the second half, assisted by Gia Hung.
Iran will compete against the winners of Thailand versus Tajikistan in the semi-finals.
