We still use the old number words from Hanzi: nhất, nhị, tam, tứ, ngũ, lục, thất, bát, cửu, thập....
In certain Sino-origin words like "thập niên". They are not common in mainstream use but most Vietnamese would still understand.
They are relegated to various Sino-Vietnamese terms (nhất thiết 一切, has a different meaning to the Chinese) and (tam giác 三角) although nhị 二 and tứ 四 have also become nhì and tư respectively and used to mean 2nd and 4th (essentially ordinal number 2 and 4) alongside nhất (first).
I'm confused. The translation hint list thập kỉ as translation for decade. Is that hint wrong?