Estimating possible tax burden in Vietnam

I've recently considered spending part of more of my retirement in Vietnam, after having looked at the Philippines for a while.

I've just begun investigating the tax situation I'd face in VN.  While I've found some info, I could use some clarification on certain issues, especially regarding anything impacted by any tax-treat rules between the U.S. and Vietnam.

My income situation is as follows:

- I get about $ 28K a month from SSDI in the U.S., which I can readily have deposited to an international bank if that's preferable to depositing it to a VN bank.

- I get about $12K/year in investment income (dividends, cap gains (average), and cap gains distributions).

From what I've read to date, I'd expect to be on a tourist visa for the foreseeable future after arriving, since I know little about retirement visas in VN.

U.S. taxation in my situation IRS federal taxation is fairly gentle due to favorable tax treatment granted to social security and investment income for retirees with modest incomes.  However, I couldn't tell whether VN is similarly considerate.

From the sites I've reviewed so far, it wasn't clear to me whether someone physically resident in VN is a "resident" for tax purposes, if the retiree doesn't have a formal retirement visa. 

It wasn't clear to me from the sites I've seen what tax level is imposed on investment income.  (one site said that business income was taxes at 5%; another said that all income was taxed according to a progressive schedule).

Would appreciate any input from the board.