I can't speak for the entirety of the country and will probably be wrong about Hanoi and Saigon which have a much greater market for foreign employees especially teachers but here in Da Nang the retirees are likely the largest segment of the expat population.
Many of these expats are retiring here because their pensions go a lot further than they would if they decided to retire in their home country. For example it is quite easy to live off of $1000 a month here, all in. This includes all bills including rent, foot and drink, internet, mobile, water, and a bit of travel. Obviously people can spend a great deal more but I personally live quite lavishly in a rather posh condo, almost never make meals at home and I keep it around $1000 a month every month.
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Many of these retirees have some sort of investment in a business of some sort here because as of now this country doesn't have any sort of retirement visa. I think this is a mistake because neighboring Thailand absolutely kicks Vietnam's ass on tourism and one of the reasons why that is the case is because they have a retirement visa program that is available for anyone over the age of 50 that can prove they have a certain amount of money. Hopefully this will change in the future.
There are options though and the "entry fee" so to speak isn't that high. If you can find a company that you actually believe in you are eligible to get an investor visa for as little as $20,000 invested into the company. Now I know that not just every Tom and Joe has 20 large lying around but honestly, if you don't have 20 grand you don't really have any business attempting to retire anyway.
One couple that I know are invested in a micro-brewery and they are silent partners. They have part ownership of the company but no say in how it is managed. This is just fine by them because they don't want to work anyway. In their case this was a wise investment because the microbrewery business is really big in Vietnam and the one they are involved in has quite a presence all over the country. So not only are they getting long term visas but their investment is also paying off. For others it might not be so lucky.
Regardless though, if someone, retired or not goes for a visa this way the only way to lose the visa is if the company becomes insolvent and closes down.
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I will say this about the retired community here and I don't mean this as an insult but rather just a statement of fact. The retirees here in Vietnam tend to be much heavier drinkers than most of the retirement world that exists in their home countries. I'm not saying they are problematic drunks because that is not the case outside of a few waste-oids here in there. The retired folks that I see at the pubs generally have their shit together and will go home relatively early in a responsible manner before they start to get in trouble or lose control of their limbs. I suppose this comes from a lifetime of wisdom gathering. However, the point remains that the retired folks here, compared to their counterparts back in the west, are a bunch of drunks :P
I claim no high ground here because compared to people my age back in the west I too am a drunk.
While I do not know everyone in Da Nang I would say that the retired folks probably constitute around 40% of the overall expat mix if I had to take a guess. They are generally affluent but probably wouldn't be considered as such in their home countries. I think this is a big part of the attraction for people trying to retire over here. Especially these days where the world is becoming absurdly expensive and almost impossible to retire from in places like USA, this is probably a very wise choice for people, especially if they didn't manage to save up millions of dollars in their work life in preparation for retirement. I think that a couple, living reasonably well, could quite easily live a good life in Vietnam for around $20,000 per year total. I honestly don't think that is possible in the west. Most simple pensions and even Social Security will cover this sort of bill, so I can understand why they would choose to move here.
I like hanging with the retired folks because when I do so I am actually one of the younger ones in the group despite being in my 40's.