Index: sites/all/modules/transliteration/data/x00.php
===================================================================
--- sites/all/modules/transliteration/data/x00.php	(revision 1107)
+++ sites/all/modules/transliteration/data/x00.php	(working copy)
@@ -31,3 +31,9 @@
   0xFC => 'ue',
   0xDF => 'ss',
 );
+
+// Overrides for Vietnamese input.
+$variant['vi'] = array(
+  0xDD => 'y', // ý
+  0xFD => 'y', // Ý
+);
\ No newline at end of file