Про що ви думаєте?
Про = about
E.g. Я думаю про кота = I am thinking of/about a cat (Accusative)
Would this sentence be the same when we use ви as a formal you or do we use ви думаєш? :)
No, it's still "Ви думаєте" :) The formal you is equivalent to the plural you in all aspects.
I remember having a problem with this as a little kid!.. I would say "Ви дуже гарна" to a woman instead of "Ви дуже гарні" and couldn't understand why my mom said it was wrong :)