Vietnamese has a "u" sound, but no equivalent of the French [y] sound (spelled "u") — it's one of the hardest sounds for Vietnamese speakers, because the ear isn't yet used to telling it apart from "ou" [u], which is much closer to the Vietnamese "u." Mixing the two turns "tu" (you) into "tout" (everything), or "rue" (street) into "roue" (wheel).
Both sounds are produced with rounded, forward-pushed lips, but they differ in tongue position. "ou" [u] pulls the tongue back — close to the Vietnamese "u." The French "u" [y] rounds the lips the same way, but pushes the tongue forward near the front teeth — like saying "i" while shaping your lips as if saying "u." That combination doesn't exist in Vietnamese, so it needs dedicated practice.
Say the Vietnamese "i," keep your tongue exactly where it is, then slowly round your lips as if about to whistle — that's the [y] sound.
Drill word pairs that differ only in this sound: "tu/tout," "rue/roue," "su/sous" — if you can't hear the difference, you'll struggle to produce it correctly.
Practice hearing the two sounds apart using minimal pairs before working on producing them, since this is a brand-new sound for Vietnamese speakers, not a variant of something familiar.
Many learners think they're producing it correctly when they're actually still closer to "ou" — recording your own voice and comparing side by side is the most reliable check.
See 5 French pronunciation mistakes Vietnamese speakers make for other errors that affect comprehension and your Speaking score just as much.
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