diff --git a/crates/proof/src/lib.rs b/crates/proof/src/lib.rs
index 36902552..a0099c66 100644
--- a/crates/proof/src/lib.rs
+++ b/crates/proof/src/lib.rs
@@ -49,6 +49,7 @@ pub mod pl;
 pub mod pt;
 pub mod ru;
 pub mod tr;
+pub mod vi;
 
 use std::collections::HashSet;
 use std::ops::Range;
@@ -581,7 +582,8 @@ fn enclosing_token(text: &str, a: usize, b: usize) -> (usize, usize, bool) {
 /// classification of the word's enclosing token, from [`enclosing_token`].
 fn spelling_issue(text: &str, a: usize, b: usize, lang: ProofLang, opts: &Options, internet: bool) -> Option<Issue> {
     let w = text.get(a..b)?;
-    if (opts.ignore_internet && internet) || is_correct_in_with(w, lang, opts) {
+    // A word with a Vietnamese-only letter (a name like Nguyễn in English text) isn't English.
+    if (opts.ignore_internet && internet) || (lang == ProofLang::En && vi::is_vietnamese_word(w)) || is_correct_in_with(w, lang, opts) {
         return None;
     }
     let message = match lang {
@@ -668,9 +670,19 @@ pub fn check_text_with(text: &str, spans: &[(Range<usize>, ProofLang)], opts: &O
             _ => merged.push((r.clone(), *l)),
         }
     }
+    // A paragraph typed in Vietnamese under the default English language is Vietnamese, which
+    // has no proofing tools: no marks (Word detects the language the same way).
+    let default = if vi::looks_vietnamese(text) {
+        for (_, l) in merged.iter_mut().filter(|(_, l)| *l == ProofLang::En) {
+            *l = ProofLang::Other;
+        }
+        ProofLang::Other
+    } else {
+        ProofLang::En
+    };
     let lang_at = |i: usize| -> ProofLang {
         let k = merged.partition_point(|(r, _)| r.end <= i);
-        merged.get(k).filter(|(r, _)| r.start <= i).map_or(ProofLang::En, |s| s.1)
+        merged.get(k).filter(|(r, _)| r.start <= i).map_or(default, |s| s.1)
     };
     let mut v: Vec<Issue> = spelling_issues(text, lang_at, opts);
     if !grammar {
@@ -686,7 +698,7 @@ pub fn check_text_with(text: &str, spans: &[(Range<usize>, ProofLang)], opts: &O
         covered = covered.max(r.end);
     }
     if covered < text.len() || merged.is_empty() {
-        langs.push(ProofLang::En);
+        langs.push(default);
     }
     langs.sort();
     langs.dedup();
diff --git a/crates/proof/src/vi.rs b/crates/proof/src/vi.rs
new file mode 100644
index 00000000..bb24b88e
--- /dev/null
+++ b/crates/proof/src/vi.rs
@@ -0,0 +1,93 @@
+//! Telling Vietnamese text apart from English, so a document typed in Vietnamese under the default
+//! English proofing language isn't covered in red underlines (as Word's "detect language
+//! automatically" does). There are no Vietnamese proofing tools yet, so Vietnamese text is treated
+//! like any language without them: no spelling or grammar marks.
+//!
+//! The test is orthographic. Vietnamese writes most syllables with letters no other Latin-script
+//! language shares: the horn letters (`ơ ư`), `ĩ ũ`, and the tone-marked vowels of the Latin
+//! Extended Additional block (`ạ ả ấ ầ … ỹ`), precomposed or as combining hook above, dot below
+//! and horn. French and Portuguese share `â ê ô à é`, Romanian `ă` and Croatian, Serbian and
+//! Bosnian `đ`, so those alone don't count.
+
+/// Does `c` occur in Vietnamese words but (practically) nowhere else?
+fn vietnamese_letter(c: char) -> bool {
+    matches!(c, 'ơ' | 'Ơ' | 'ư' | 'Ư' | 'ĩ' | 'Ĩ' | 'ũ' | 'Ũ' | '\u{1EA0}'..='\u{1EF9}' | '\u{0309}' | '\u{0323}' | '\u{031B}')
+}
+
+/// Is `word` written with a Vietnamese-only letter (`Việt`, `ơn`)? Such a word is never an
+/// English misspelling.
+pub fn is_vietnamese_word(word: &str) -> bool {
+    let combining = |c: char| matches!(c, '\u{0300}'..='\u{036F}');
+    // Two marks stacked on one letter (decomposed `ế` = e + circumflex + acute) are Vietnamese too.
+    word.chars().any(vietnamese_letter) || word.chars().zip(word.chars().skip(1)).any(|(a, b)| combining(a) && combining(b))
+}
+
+/// Does a paragraph read as Vietnamese? At least two of its words (any, in a heading of up to three
+/// words) carry a Vietnamese-only letter, and they are at least a sixth of its words: Vietnamese syllables without
+/// such a letter (`anh`, `con`, `ba`) are common, while an English paragraph quoting one
+/// Vietnamese name stays English.
+pub fn looks_vietnamese(text: &str) -> bool {
+    let mut total = 0usize;
+    let mut marked = 0usize;
+    for w in text.split(|c: char| !(c.is_alphanumeric() || matches!(c, '\u{0300}'..='\u{036F}'))).filter(|w| !w.is_empty()) {
+        total += 1;
+        if is_vietnamese_word(w) {
+            marked += 1;
+        }
+    }
+    marked > 0 && (marked >= 2 || total <= 3) && marked * 6 >= total
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn vietnamese_words() {
+        for w in ["Việt", "ơn", "những", "Ước", "mũi", "Nghĩa", "tie\u{0302}\u{0301}ng", "nho\u{031B}"] {
+            assert!(is_vietnamese_word(w), "{w}");
+        }
+        // Shared with French, Portuguese, Spanish: not enough on their own.
+        // Romanian ă and Croatian đ are Vietnamese letters too, but not Vietnamese-only.
+        for w in ["café", "être", "côte", "água", "señor", "anh", "the", "mănânc", "acasă", "Đurđa", "đak", "đi"] {
+            assert!(!is_vietnamese_word(w), "{w}");
+        }
+    }
+
+    #[test]
+    fn paragraphs() {
+        assert!(looks_vietnamese("Tôi yêu tiếng Việt và những bài thơ của nó."));
+        assert!(looks_vietnamese("Hà Nội"));
+        assert!(looks_vietnamese("Việt"));
+        // English quoting one Vietnamese name, and French, stay as they are.
+        assert!(!looks_vietnamese("The meeting with Mr Nguyễn is on Tuesday at the main office."));
+        assert!(!looks_vietnamese("Le café de la côte est très bon."));
+        assert!(!looks_vietnamese(""));
+        assert!(!looks_vietnamese("anh em"));
+        // Romanian and Croatian paragraphs, full of ă and đ, aren't Vietnamese.
+        assert!(!looks_vietnamese("Mă duc acasă să mănânc cu fratele meu, că e târziu."));
+        assert!(!looks_vietnamese("Đurđa i Đorđe su se vratili kući, a Đakovo je daleko."));
+    }
+}
+
+#[cfg(test)]
+mod checker_tests {
+    use crate::{Options, ProofLang, check_text_with};
+
+    #[test]
+    fn vietnamese_paragraphs_under_english_get_no_marks() {
+        let text = "Hôm nay tôi đi học, trời rất đẹp và mát mẻ.";
+        let spans = [(0..text.len(), ProofLang::En)];
+        assert!(check_text_with(text, &spans, &Options::default(), true).is_empty());
+        // No language at all (the document default) behaves the same.
+        assert!(check_text_with(text, &[], &Options::default(), true).is_empty());
+    }
+
+    #[test]
+    fn english_keeps_its_marks_and_skips_vietnamese_names() {
+        let text = "We met Nguyễn yesterdy at the office.";
+        let issues = check_text_with(text, &[(0..text.len(), ProofLang::En)], &Options::default(), false);
+        let flagged: Vec<&str> = issues.iter().filter_map(|i| text.get(i.start..i.end)).collect();
+        assert_eq!(flagged, ["yesterdy"]);
+    }
+}