Vietnam isn't exactly the most welcome place if you want a hookup. Decades of warring with the westerners, and then being communist for the rest of the country's life (which is by default anti-western) doesn't give Vietnam a West-loving impression.
Vietnamese don't have a grudge
On a personal level, not many Vietnamese will hold grudge against people from Europe or America for what their governments might have done half-a-century ago. Political differences don't matter much. However, what matters is the by-product of different ideologies, namely, the difference in values and upbringing.
How is Vietnam different
Even without all the political toil, Vietnam is much more traditional than most countries in Europe, and especially the US. That means less chance of quick hookup
than usual and less chance of doing your Vietnam hookup online.
The Vietnamese are much more reserved even than their neighbors, not in the least due to Vietnam's inner policies. That means you'll be less lucky to hookup a woman at the, say, night club, and then spend a night with her at home, for two reasons:
1. The Vietnamese aren't brought up this way;
2. There aren't many night clubs in Vietnam, and it's half-intentional
Cute Vietnamese Girls Who had Crush on Me, Walking around Hanoi, Vietnam
How do Westerners do Vietnam hookup then?
It's fairly easy actually. The Vietnamese may not be eager Internet users, but they still use the Internet. And the Internet has a way to tell if people are mentally compatible.
And that doesn't only mean similar interests, but also how well you'll understand each other. The majority of the frequent Internet users are fairly 'compatible'. The dating apps, however, allow you to pick the people most alike to you. And they will have a sizeable Vietnamese community in them, even if it's not as numerous as some other.
Picking an app is all there's left to do for you.
There are several you might consider. The popularity of the app, funnily enough, is one of the less important aspects you have to take into account. If your choice is a fairly popular app overall, it won't matter. You'll find a few people to your liking.
What you need to consider
What you have to consider is a couple of subjective aspects:
1. The comfort of the app in question;
2. How quickly you can hookup people with it;
3. Are there many people like you on the app?