In the men's 100m backstroke qualifying round, Le Nguyen Paul came ninth in his heat in a time of 56.95sec. He is No 44 among 64 rivals in this category.
Le, born in 1992 in the US, is swimming for the first time in a Vietnam's colours.
He will compete in the 50m and 200m backstroke in the coming days.
In the men's 200m freestyle qualifying round, SEA Games winner Hoang Quy Phuoc fell short of his performance in Singapore two months ago, finishing eighth in his heat with a time of 1min 54.31sec, ranking 63rd among 80.
In Singapore, Phuoc set a new Games record of 1:48.96.
The Danang-born athlete will also be competing in the 100m freestyle and 100m butterfly events.