Apple iPad iOS 5.1 - Manual (Page 135)

Vietnamese
Touch and hold a character to see the available diacritical marks, then slide to choose the one
you want.
You can also type the following key sequences to enter characters with diacritical marks:
aa—в В (a circumflex)
 aw—ă (a caron)
 ee—ê (e circumflex)
 oo—ô (o circumflex)
 ow—ơ (o hook)
 w—ư (u hook)
 dd—đ (d dash)
 as—á (a acute)
af—а
В (a grave)
 ar—ả (a question mark)
 ax—ã (a rising accent)
 aj—ạ (a drop tone)
135 Appendix B International Keyboards