What this means for learners
Vietnamese and Lao share 23 sounds — roughly 51% of Lao's inventory overlaps with Vietnamese. Shared sounds are ones a speaker already knows from their native language and will generally produce and perceive accurately without explicit training.
The 19 sounds found only in Lao represent the greatest pronunciation challenge for Vietnamese speakers. The adult brain tends to map unfamiliar sounds onto the closest native equivalent — a process that produces the characteristic "accent" of a second-language speaker. Learning to hear and produce these sounds as distinct requires focused ear training, not just repetition.
Conversely, Vietnamese has 22 sounds not used in Lao. Native Lao speakers learning Vietnamese will face the mirror-image challenge with these sounds.
Phoneme inventories from PHOIBLE. Data reflects one documented inventory per language; some variation exists across dialects and sources.