Apple iPod touch iOS 5.1 - Manual (Page 134)

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—â
aw—ă
Â
as—б В
af—а В
ar—ả Â
ax—г В
aj—ạ Â
dd—đ Â
ee—к В
oo—ф В
ow—ơ Â
w—ư Â
134 Appendix A International Keyboards