Vegeta is Majin Vegeta at Buu Saga
Goku is when Goku was fighting with Majin Vegeta, except assuming he's alive.
Rules: Goku cannot transform into SSJ3. Both characters start in SSJ2.
Goku's brain is replaced by Batman's. Batman has knowledge of what Vegeta is capable of doing but otherwise is completely himself in terms of mind and knowledge.
1 day prep time. Goku/Batman can go anywhere on DBZ planet Earth he wants during that time (this battle assumes that he has access to a personal aircraft)
Setting during battle: same place as Majin Vegeta vs SSJ2 Goku.
Fight to death/incapacitated