I have a retirement visa here in Thailand. I want to work in Vietnam for several months, maybe a year, returning to Thailand maybe 1-2 times/month. Is that OK?

I assume you have an extension of stay from immigration not a visa from an embassy,

As long as you have a valid re-entry permit to keep your extension valid it is not be a problem to work anywhere outside the country.

I presume you meant to say you have an extension of stay, based on retirement, obtained from a Thai immigration office.

You can achieve what you wish.

Make sure you have a a multiple re-entry permit.

Yes, exactly, extension of stay. Thanks for the responses. I do indeed have a multiple reentry permit.

Just wanted to be sure that the extension of stay doesn't REQUIRE one to be here so many days of the year.

You do not have to be in the country any number of days to keep your extension valid.

There is only one day you have to be here and that is the date you apply for your next extension.