Looking for a teaching position in Hanoi

If you wish to work in language centers, you will need some type of TEFL/TESL certificate regardless of your high level of experience.  It may be legally sufficient on paper to have a teaching degree and obviate the TEFL requirement for a work permit but you don't know which bureaucrat will be approving your application.  If your goal is semi-retirement a TEFL course is still the way to go.  There are several options but I would recommend taking the CELTA in Vietnam as it seems to be the most respected and gives you a chance to get your feet on the ground for a month while taking the course.

If you want to teach full time you should apply to international schools in Hanoi where the language of instruction is English.  The school can let you know what documents will be required.  Two salaries at international schools should afford a quite high standard of living and a possibly high savings rate.