The opponents, who were in semifinals for the first time in the tournament's history, entered the game confident with nice ball possession and accurate passes.
But Vietnam quickly took over and the game was mostly played in Cambodia's half.
The main strikers Tien Linh and Duc Chinh, Vietnam's top scorers at the tournament, added three more goals to their tally, while Chinh contributed to Yue Safy's own goal that allowed Vietnam to lead 4-0 in the 68th minute.
Even a last minute penalty for Cambodia failed to disturb the score as custodian Nguyen Van Toan parried away Keo Sokpheng's shot with minimum fuss.
Vietnam are potentially one match away from making SEA Games history, with Indonesia awaiting them at the final hurdle on Tuesday.