I don't think Vietnam has any McDonalds - didn't see any anyway.
We did see one KFC in Hanoi, but they didn't seem to have much else in the way of fast food.
We sometimes break-up our current diet of Chinese food with the odd trip to McDs - sometimes you just need something familiar.
Whenever i travel, i make sure i eat at McDonalds and see the difference. It's my vow to eat at McDonalds in my travels (Paris, Rome, Singapore, US, Baguio City, Cebu City, etc). And here in Manila, we have McSpaghetti.