FYI, vang is typically only used in Northern Vietnam, most people in southern vietnam (and Vietnamese internationally) use "ya"
Phonetically to an English speaker, it sounds like "ya" but written as "dạ".
Nguoi mien nam noi "da" nhung ma nguoi bac noi "vang"
Shouldn't "OK, thanks" be accepted as well?
It says that "vâng" is a polite form of "yes," so I'm guessing "okay" would be marked wrong because you most likely wouldn't use it in a more formal setting in English.
Northern isn't the only spoken vietnamese in the world, duolingo. Come on now