diff --git a/lisp/language/viet-util.el b/lisp/language/viet-util.el
index d672f77f250..33663843d8a 100644
--- a/lisp/language/viet-util.el
+++ b/lisp/language/viet-util.el
@@ -220,10 +220,13 @@
     )
   "Alist of Vietnamese characters vs corresponding `VIQR' string.")
 
-;; Regular expression matching single Vietnamese character represented
-;; by VIQR.
 (defconst viqr-regexp
-  "[aeiouyAEIOUY]\\([(^+]?['`?~.]\\|[(^+]\\)\\|[Dd][Dd]")
+  "[aeiouyAEIOUY]\\([(^+]?['`?~.]\\|[(^+]\\)\\|[Dd][Dd]\\|\\\\[(^+'`?~.d\\]"
+  "Regular expression matching VIQR representation of a single character.")
+
+(defconst viqr-punctuation-regexp
+  "[(^+]?['`?~.]"
+  "Regular expression matching punctuation chars that must be escaped in VIQR.")
 
 ;;;###autoload
 (defun viet-decode-viqr-region (from to)
@@ -257,6 +260,7 @@ positions (integers or markers) specifying the stretch of the region."
   (save-restriction
     (narrow-to-region from to)
     (goto-char (point-min))
+    (replace-regexp-in-region viqr-punctuation-regexp "\\\\\\&")
     (while (re-search-forward "\\cv" nil t)
       (let* ((ch (preceding-char))
 	     (viqr (cdr (assq ch viet-viqr-alist))))
diff --git a/test/lisp/language/viet-util-tests.el b/test/lisp/language/viet-util-tests.el
new file mode 100644
index 00000000000..be6dbe5c07e
--- /dev/null
+++ b/test/lisp/language/viet-util-tests.el
@@ -0,0 +1,37 @@
+;;; viet-util-tests.el --- unit tests for viet-util.el  -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2025 Free Software Foundation, Inc.
+
+;; This file is part of GNU Emacs.
+
+;; GNU Emacs is free software: you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; GNU Emacs is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.
+
+;;; Commentary:
+
+;; Unit tests for lisp/language/viet-util.el.
+
+;;; Code:
+
+(ert-deftest viet-util-test-viqr ()
+  "Test bug#80039."
+  (let ((viqr-text "O^ng te^n gi`\\? To^i te^n la` Tra^`n Va(n Hie^'u\\.")
+        (viet-text "Ông tên gì? Tôi tên là Trần Văn Hiếu."))
+    (with-temp-buffer
+      (insert viqr-text)
+      (viet-decode-viqr-region (point-min) (point-max))
+      (should (equal (buffer-string) viet-text))
+      (viet-encode-viqr-region (point-min) (point-max))
+      (should (equal (buffer-string) viqr-text)))))
+
+;;; viet-util--tests.el ends here