There is no linkage between 'retirement' and residing in VietNam for which you need a Residency Permit.
However, in order to work legally you require an additional Work Permit.
However, as a potential retiree you need to put some plans in place.
Working banking account: You need a InterNet Banking account which has minimal charges for using an ATM. You don't want to keep too much money in VN as the monetary policy really is a hassle with foreign denominated currency.
In the case of divorce the VNese law provides for 50-50 assets split. Keeping your money in another country protects your assets.
Intermediate Bank: It is often useful to have a savings account, in a separate account, into which your government/pension payments are made. Handy for settling accounts in the US. Should have InterNet access.
This account should have on-line transfer facilities to the Working Account. This greatly reduces opportunity for fraud.
Accommodation Address: It is always handy to have an 'accommodation address', other than a Box, through to receive mail, etc.
US Pensions: The US pension should be deposited in to the Intermediate account from which you transfer to your working account.
The US likes to know you are alive. If you tell them (even through tax filings) they will make you report to the Embassy/Consulate every month. It can be really inconvenient to have to travel each month if you live in the countryside.
Assignments/Will: The VN government will inform the government that issued the passport on which you entered VN when you die. The only time they don't advise another government is if you have VN citizenship (as well).
When a government knows you are dead they usually make sure any payments cease.
If you have people you wish to leave assets to in the US things ar easy. However, should you wish to give someone in VN your assts needs planning.
In my case I have the recipient of the assets and a very trusted friend, and they don't know each other. The recipient has all my account information, the other my passwords.
If you develope a relationship with a VNese person, you must retain control of your assets. This means putting your name on the Land User Certificate (cannot be pledged or transferred without your consent), likewise with real property - except you can be shown as an owner.
Why, you ask? Control. You retain control of your assets. Even if you have a VNse wife, retain control. There are many stories of VNese being kidnapped or blackmailed so their (rich) Foreign family member can pay the money. If you don't male provision for your being a signing party on any transaction you might find you suddenly have no assets.
Note: Once the US Government knows you have died, the information is published and, usually, all your bank/investment accounts are frozen.
Check your PM.