On the page vi:Bánh phở, clicking the "Sửa" button (for VisualEditor) gives me the content of an old revision of de:Stadtbefestigung Koblenz. (Looks like this one.)
Topic on VisualEditor/Feedback
Sorry about this; we've found it in a couple of places (it's a cache corruption bug on Parsoid - you're getting the 'right' oldid (11666092) but on the wrong project, so you have the text of https://de.wikipedia.org/w/index.php?title=Stadtbefestigung_Koblenz&oldid=11666092 when you're editing https://vi.wikipedia.org/w/index.php?title=B%C3%A1nh_ph%E1%BB%9F&oldid=11666092 - I think it'll be fixed very soon.