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.)
However, once you identify your list of target colleges, you would be wise to check each Web site carefully (or call admission offices) because policies can vary from school to school. You may occasionally see that a college (e.g., Carnegie Mellon) asks for a TOEFL score from any candidate whose first language isn't English. You may also find that, at some colleges (e.g., Amherst), students who score below a particular cut-off on the Critical Reading section of the SAT are asked to submit a TOEFL score, too.
If only one or two colleges on your list seem to want a TOEFL from you, and if your Critical Reading score is well in the ballpark for these schools, you can probably get a waiver on the TOEFL if you so choose. Just make sure you get it in writing (e-mail is fine) and keep a record of it in your files.
I know that such inconsistencies can be a pain, but this whole process is so full of them that this may be good training for what lies ahead. :-(