Question: I am from Vietnam and had lived in the U.S. for more than 10 years and am a U.S. citizen. Given that, do I still need to take the TOEFL?
The short answer is "No," but there may be some exceptions. The usual rule of thumb is that, if English has been your primary language of instruction throughout high school (for at least four years), then the TOEFL is not required. In fact, most colleges would totally disregard a TOEFL score from someone who has been in the U.S. as long as you have. (And citizenship doesn't factor into the TOEFL issue at all. The key is the amount of time you've spent studying in English.)