The upcoming release of iOS 4.2 finally brings Vietnamese input to iPad, with a keyboard that uses the standard Telex input method, together with autocorrect. :-)
Available since iOS 4.1 on iPhone, a new feature of Vietnamese input on iPad brings the improvement that the Telex input method works anywhere in the text, rather than just on the current Vietnamese syllable being typed. In other words, once Vietnamese text is input, the user can back up anywhere, and still be able to use Telex to adjust the text.
Users not already familiar with the Telex input method can press and hold any key, and then select the appropriate Vietnamese character with diacritic(s). For an explanation of the Telex input method, please see a previous post.