"How are you going to choose?"
Translation:¿Cómo vas a elegir?
Why can't I use va (with an understood usted)? Or even van (with an understood ustedes)?
"tú" and "tu" aren't the same thing; the second one is a possessive determinand in second person ('your').
Most of the times the pronoun is not included unless you want to add emphasis. The inflectional verb (vas) does all the job to specify the pronoun. Otherwise, it's plainly redundant and sounds like an imcomplete sentence.