But the defending champions are keeping the belief that they can pull off a comeback ahead of Sunday's return encounter at Singapore's National Stadium.
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After falling two goals behind courtesy of a first-half brace from Thai captain Chanathip Songkrasin, Vietnam produced a much-improved display in the second 45 on Thursday and were unlucky not to have at least pulled one back after talismanic playmaker Nguyen Quang Hai was twice denied by the woodwork.
"For sure, 2-0 is really not an easy scoreline to overcome but it is also not impossible," said Vietnam midfielder Luong Xuan Truong.
"I think Thailand did play better than us in the first half of that first leg and had more chances, but we got better in the second half. We created more chances and were really unlucky not to have scored but this is football, so we must accept it.
"Tomorrow (Sunday), we will try our best for a good result with the aim being to at least score two goals."
Adding further spice to what was already to be a tense affair between two fierce rivals, the opening encounter threatened to boil over on numerous occasions and was marred by confrontational scenes at the final whistle.
Thailand defender Theerathon Bunmathan was identified as the main culprit as his celebratory actions were deemed as having been provocative as the victors walked past the Vietnam bench to acknowledge their supporters, while goalkeeper Chatchai Budprom could also be pointing to the side of his head while gazing at the opposition -- in what could be construed as a gesture asking the recipient to think again.
Visibly still displeased, Vietnam coach Park Hang-seo said: "After the match, when Thailand came through, I only went to him (Theerathon) to talk to him about his actions but he wasn't polite and provoked me.
"I can understand their reaction given this was the first time they have managed to beat us in four years, but I will never teach my players to perform such actions with the coach of any opposition.
"I think all these issues also stem from the referee not having strong actions with the players and the opposition coach. Having reviewed the match many times, I cannot understand some of the refereeing decisions and I think all Vietnamese fans have the same feeling.
"After losing the first leg 2-0, it won't be easy in the second leg but we must try harder. My only request to the players is to do their jobs well and keep calm."