Phuoc finished 59th out of 115 participants in the men's 100m freestyle on August 5. Still recovering from a back injury, he came third in heat 6 with a time of 51.21sec, which was far off from the times of swimmers who secured tickets to the Brazil Olympics.
Phuoc will have one more chance at an Olympic slot in the 100m butterfly tomorrow.
Khoi swam in only one event, the men's 200m individual medley in Russia, and finished at 31st among 47 swimmers.
For today, the fifth day of competition, the only Vietnamese to show will be Le Nguyen Paul competing in the men's 200m backstroke.