The Vietnam Anti-Doping Agency informed Wednesday about the new three cases: one male and two females.
All five belong to Vietnam's track and field team, two of them gold medallists.
A source who did not wish to be named told VnExpress that one won SEA Games gold for the first time while the other is a veteran athlete who had won high achievements at Asian tournaments.
The athletes' identities are expected to be announced by the World Anti-Doping Agency.
The five athletes would undergo a second doping test, with samples taken at a laboratory in Thailand next month.
The athletes have said they've not deliberately used any stimulant, and their positive tests could be linked to their diets or use of antibiotics.
The names of athletes suspected of doping have been removed from a list to receive investments for ASIAD 2023 and Summer Olympics 2024. They may also have their SEA Games gold medals revoked and face long-term sporting bans.
Vietnam has recorded 19 cases of doping since SEA Games 2003. Most of the cases have stemmed from a lack of knowledge and minimal anti-doping skills because the drugs used are common and easy to detect, like diuretics or boosters.
The 31st SEA Games took place in Hanoi and neighboring provinces from May 12 to 23, 2021 with 40 sports, 526 events with over 10,000 athletes, officials and referees from 11 Southeast Asian countries. Vietnam participated with a contingent of 1,341 who won 205, 125 silver and 116 bronze medals to finish first in the overall ranking.