Apple iPhone iOS 5.1 - Manual (Page 172)

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 Â (a caron)
as—б В (a acute)
af—а В (a grave)
ar—ả Â (a question mark)
ax—г В (a rising accent)
aj—ạ Â (a drop tone)
dd—đ Â (d dash)
ee—к В (e circumflex)
oo—ф В (o circumflex)
ow—ơ Â (o hook)
w—ư Â (u hook)
172 Appendix A International Keyboards