"When are you going to ask them about their vacation?"
Translation:¿Cuándo les vas a preguntar sobre las vacaciones?
isn't it okay to say "cuando vas a preguntarles sobre las vacaciones" and attach the "les" to "preguntar"
I'm used to being very formal, so I used "va" instead of "vas." Why is this incorrect?
The question itself seems more informal to me than formal. After all, would you walk up to your boss, your teacher, or someone that you don't know very well and ask, "When are you going to ask them about their vacation?" That could be blunt in some cases and rude in others.