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.

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