Alternet has an article on five popular destinations. For VietNam it says:
The Good: VietNam is just emerging as an attractive place for Americans to retire after decades of negative associations. The low cost of living and beautiful countryside are enticing for budget-conscious retirees who like adventure.
The climate is quite varied throughout the country, from humid jungles to cool mountaintops and sun-drenched beaches. Nha Trang, with its stunning coastal landscape is a popular destination, as is Hoi An, famed for its lively culture and excellent cuisine. The lovely island of Phu Quoc is also quite popular.
The Bad: Pollution is a major complaint for expats in VietNam, and it is not uncommon to see people walking down the street wearing masks. Retirees can also expect daunting paperwork and the hassle of maintaining a tourist visa, since Vietnam does not offer a retirement visa.
Property ownership restrictions can pose problems, and many find the chaotic traffic (VietNam has one of the highest road fatality rates in the world), noise and the lack of Western influence to be drawbacks. Some expats experience the cultural divide as exhausting to overcome, and find it somewhat difficult to forge relationships with locals. Many end up retreating into an “expat bubble.”
Lynn Stuart Parramore, AlterNet