Is my Vietnamese passport holder wife eligible for a 30-day extension on a 45-day visa exempt stay in Thailand?

Don't forget the visa exempt is 45days now but will end 30th march 2023 and return to 30 days as per normal smart people are using this to their own advantage at moment by enter thailand for 45days visa exempt than visit a border country short time than re enter thailand stamp in for another 45 days visa exempt and you can extend once for another 30 day 1900baht which will get a total of 120 days in thailand plus your time in a border country like Vietnam or Cambodia maybe laos like thai girls say up to you

You can look at the options to apply for a tourist visa before you get here (would give her 60 days - extendable once for 30 days). Or the multiple entry tourist visa that is valid for 6 months from issue and on each entry she’d get 60 days. Timing things write you can get nearly 9 months off that visa type.

It doesn't matter that she came from the USA or have legal residency there. Visas and extensions are based on passport nationality. She won't qualify for extensions since she entered on a Vietnam passport.

No. Vietnam passport holders enter under a bilateral agreement called a visa waiver. The difference from a “normal” visa exempt is they cannot get extensions off it.