Going from Japanese to Vietnamese (Tiแบฟng Viแปt) has a composite linguistic distance of 86% (heuristic estimate). Below is the dimension-by-dimension breakdown.
Shared vocabulary, cognates, and loanwords.
Morphology, syntax, word order, cases.
Shared sounds and phoneme inventory.
Japanese โ Latin
Japonic โ Austroasiatic
The 5 languages with the lowest linguistic distance from Japanese:
A quick 3-question check between Japanese and Vietnamese. No signup.