If you’re not, then Vietnam Cupid is probably your best bet.
It’s the site I always use when I fly in, I setup dates with around 6-7 girls a few weeks before hand, and end up sleeping with at least half. Do keep in min that some girls will flake (perhaps 10-20%), but that’s normal.
Around 99% of are regular Vietnamese girls, and 1% are hookers. Just setup a profile with 3-4 pictures, be friendly and just casually ask if they would like to meet up for a coffee or dinner when you land. Just set up a number of dates and you will be fine. Even though Vietnam is considered South East Asia, and thought of in the same boat with the likes of Thailand or Indonesia, the country has far more in common with China than those regions.
This is due to 1) high population 2) language 3) it’s closed-off society and should approach the country with the same idea.
Which is great because that’s how you know Vietnamese girls are in love with you and can do whatever you want with them.