Following DPR Korea’s win over China earlier in the day, the defending champions needed to see off the hosts to lift the trophy and they did so in style.
Mai Kyokawa broke the deadlock nine minutes before the break and doubled her tally six minutes later to give Japan a half-time cushion they never relinquished.
Yokoyama then helped herself to a treble in the second period as they notched a fourth consecutive triumph to go along with their opening draw against China.
Japan dominated the first 45 minutes and Yokoyama tested Tran Thi Kim Thanh after just five minutes but the Vietnam keeper responded with a fine reflex save.
Yokoyama then shot narrowly over and Thi Kim Thanh comfortably gathered Chika Kato’s effort from long range as the hosts continued to frustrate Japan, who had plenty of pressure but could not breach the massed ranks of a determined Vietnam rearguard.
That changed in the 36th minute, though, when they finally made the breakthrough. Yokoyama curled in a delightful cross from the right towards the back post and although Kato saw her header come back off the underside of the bar, Kyokawa was on hand to nod in virtually on the line.
They drove home their advantage in the early stages of the second period.
First, Yokoyama added the third goal just a matter of seconds after the restart and then in the 52nd minute, Akane Saito was brought down in the box by a Vietnam defender’s clumsy challenge from behind.
The full-back picked herself up to stroke the ball into the bottom corner past the diving Thi Kim Thanh.
Kyokawa burst into the box to latch onto a pass on the hour mark but her low shot was held by the keeper while at the other end, Do Thi Yen’s looping shot was caught by Sakiko Ikeda.
Kyokawa then side-footed Haruka Hamada’s cross from the left past the near post first-time and substitute Naoko Wada saw a left-foot shot from Saito’s pass whistle just the wrong side of the upright from 20 yards in the 73rd minute.
Yokoyama added the fifth goal in the 83rd minute when she raced clear before rounding Thi Kim Thanh and slotting the ball into the empty net and then completed her hat-trick three minutes later, prodding home from close range following a goalmouth scramble.