I spent 4 months in Vietnam in 2007 in HCMC. Rent was $300/mo for a sweet pad down the street from the Ben Thanh market. Food was $400/mo, and I didn't cook much at all because street food is cheap and delicious.
However, I'm not sure how this would work out long term, like decades. I couldn't commit to living in a country like that full time. The language barrier would be overwhelming, and I did get tired of friends vanishing after just a few weeks. I'm a het-dude, and the local women I dated really wanted families asap and I wasn't ready -- there are a lot of highly successful women there who I got on with w/o much of a culture barrier since they had studied in US/Europe; I wasn't there doing the creepy-US-dude-looking-for-boomboom thing. Not sure I could have made a life for myself there and been happy into old age. I can't imagine what long term healthcare would be like into my 80's as an expat in Vietnam.