Going from Vietnamese to Malay (Bahasa Melayu) has a composite linguistic distance of 75% (heuristic estimate). Below is the dimension-by-dimension breakdown.
Shared vocabulary, cognates, and loanwords.
Morphology, syntax, word order, cases.
Shared sounds and phoneme inventory.
Latin → Latin/Arabic
Austroasiatic ↔ Austronesian
The 5 languages with the lowest linguistic distance from Vietnamese:
A quick 3-question check between Vietnamese and Malay. No signup.