First be sure you have the tiếng việt keyboard selected.
On my Iphone I just hold down the key and it pulls up all the options for that letter.
I imagine it's the same for iPad.
22 เมษายน 2014
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In case you want to know how to write vietnamese on the vietnamese keyboard, follow this:
Dấu sắc: press "s" after you type the vowel take it
Dấu huyền: f
Dấu hỏi: r
Dấu ngã: x
Dấu nặng: j
This is for unicode