English to Vietnamese or Vietnamese to English is harder than, say English to Spanish or Arabic because the sounds are so very different - I am sure I simply can’t hear some of the different sounds in tonal languages, and had a friend who moved here from Taiwan when she was so young she learned better in English than Taiwanese but still she could not hear the difference between ear and year.

I don’t think it’s impossible but do think it’s unusual. My dad was bilingual English and Spanish and I wish my parents had done the “one speaks English one speaks Spanish” language immersion but we only spoke English at home.