McDonalds food in Vietnam is quite different to what they sell in USA (as it is in Australia). I guess that's why the locals like it and the tourists not so much. I googled their Vietnam menu and it incudes items with a local influence, like fried chicken with rice, savory spaghetti, fish sauce chicken wings, iced milk coffee (
cà phê sữa đá), salted calamansi apricot juice, and matcha green tea McFlurries.
McDonalds apparently has a business model that allows them some flexibility for local tastes. As an example, they smartened up their format for Australia when they realised that the same menu they sell in America wouldn't cut it here. They offered better quality burgers and a range of healthier options on the menu, as well as decent coffee. They even changed the name to Macca's. Aussie Macca's isn't food I'd normally buy, but apparently it's more nutritious and tasty than the muck they sell in their American restaurants.
Plenty of American chain food brands failed in Australia because Aussies wouldn't eat or drink the same crap they were dishing up to their American customers. Starbucks, Taco Bell, Wendys, all set up in Australia with their tried and proven business model that worked in USA, and failed, or at least didn't take off like in USA. Australians wouldn't eat or drink what they were offering and the companies weren't smart enough to know why, or what to do about it.
This video covers the Aussie Macca's story pretty well. It seems McDonald's has done something similar in Vietnam: