Midfielder Momoko Tanikawa latched onto a Hana Takahashi ball and finished coolly in the 61st minute to break the deadlock. A powerful header from substitute Kiko Seike added the second in stoppage time.
The Nadeshiko, looking to reclaim the title they won back-to-back at the 2014 and 2018 Asian Cups, dominated possession but were denied more goals by a combination of resolute defending and Taiwan goalkeeper Wang Yu-ting.
Taiwan, three-times Asian champions in the late 1970s and early 1980s, parked all 11 players behind the ball for most of the game and had no attempts on goal.
Vietnam took the lead after half an hour when Nguyen Thi Bich Thuy, who had hit the crossbar earlier, sneaked up behind Indian defenders to find Thai Thi Thao, who set up Van Su for a curling finish into the top corner.
India equalised in the second half when substitute Sanfida Nongrum chipped a left-footed shot over the keeper, scoring seven minutes into her international debut.
But the night ended in disappointment for India, as Van Su's left-footed attempt nutmegged a defender to score the winner for Vietnam four minutes into added time.
Vietnam, quarter-finalists in the last tournament, will play Taiwan on Saturday, when face two-time champions Japan at the Perth Rectangular Stadium.