Vietnam will occur naturally in your My Team Mode and Driver Career (once in F1), as the third race in the Championship Series.
This can also be done by toggling to "Solo" from the home screen, then selecting "Grand Prix".
--> Choose your Series, Team, and Driver
to select the Vietnam - Hanoi Circuit track, which is the 4th selection, then hit
--> In "Weekend Structure", make sure your RACE DISTANCE is set to AT LEAST 25% (which is 14 Laps)
--> Set your AI Levels, Assists, Weather, and if you want to Practice or do Qualifying (which does not make any difference to the achievement)
I chose not to qualify, and i lined up in 2nd position, and pitted at Lap 7. Vietnam is a pretty fun circuit that can really annoy you with the zig zagging, and that one spot i always smashed my wing (xD), so you can turn Vehicle Damage off in your "Gameplay Settings" if you dont want to put up with that.
to Start Session
You do not have to win, just finish the race, and the reward is yours