From what I understand, Vercingetorix was an Arverni nobleman and therefore I'd assume after his return from exile he would be crowned at Gergoiva, but he was instead crowned at the neighboring, and Roman-sided Aeudi's capital of Bibracte. Why is this?
Vercingetorix wasn't crowned at Bibracte--other than Caesar's statement that he was hailed as king by his followers and Florus' use of the title king when speaking of him we don't know whether he was actually conferred monarchy or not. I find it unlikely that he was every actually made king, as Caesar doesn't call him that and his father had been put to death, despite already controlling Gaul, for trying to make himself king.
Vercingetorix was made commander of the Gallic armies in revolt at Bibracte in 52. Very different. Vercingetorix had returned from his exile in Gergovia some time before, and had begun the revolt from there. By the time Vercingetorix was at Bibracte the Aedui were no longer a Roman ally but had joined in the revolt--Caesar says that Vercingetorix was made commander during a council of all the Gauls, which had been summoned at Bibracte.