How do you say donde vives in a formal way?

The most formal way to say "where do you live?" in Spanish is:

¿Dónde reside usted?

Here's why this is the most formal:

* Reside is a more formal verb than "vivir".

* Usted is the formal "you" in Spanish.

Here are some other options that are less formal, but still polite:

* ¿En dónde vive usted?

* ¿Cuál es su domicilio?

The choice of which phrase to use depends on the context and the level of formality required.