You can't compare them directly, but not necessarily for the reasons you think. Vietnam has a population density of 290 people per square km, USA has 34 people per square km. Their entire economy is based on tourism and their international arrivals per capital is only about 1/3 lower than the USA.
THey have some population and demographic factors which mean they should fair better, but others that mean they should fare worse, if they'd allowed the virus to spread naturally.
The real difference is what I mentioned earlier. Vietnam were already extremely well prepared because they were already scared of a future SARS outbreak so they jumped into action from day 1, which is the best time to stop an outbreak by nipping it in the bud before it grows.
In the early stages Europe and the USA did nothing because they assumed it was going to be another regional outbreak which wouldn't effect them. They've complained in hindsight that the WHO didn't warn them, but they were told about human-human trasmission in January and then did nothing for 2 months because it was a hoax and it'll dissapear like a miracle.
Western countries allowed it to spread naturally, so unlike vietnam who stopped it through effective action, the spread is dicatated by demographics and population.