With Open the Vaults, if an aura comes back that could legally attach to an opponent’s permanent but not anything you control (let’s say Daybreak Coronet), does it have to come back attached to that permanent? If there’s another aura that could attach to a creature you control coming back from Vault (Rancor, could Rancor & DC attach to the same creature as Vault resolves? (I think the answers will be yes, and no. Just need clarification in case this happens again).
1) Open the Vaults effect is not optional. If an aura can enchant something, it must enchant something. If that happens to be your opponent's object then that is what it needs to enchant.
2) No. Auras being returned cannot be attached to anything that returns at the same time as themselves (which means Rancor could not attach to an Eidolon of Blossoms for example). In the same vein, Daybreak Coronet can only be attached to something that *already has* an aura attached to it because, at the time you make the choice of what to attach it to, Rancor is not on the field yet. You can't use Rancor as the "other" aura for Coronet to attach to something.
2) No. Auras being returned cannot be attached to anything that returns at the same time as themselves (which means Rancor could not attach to an Eidolon of Blossoms for example). In the same vein, Daybreak Coronet can only be attached to something that *already has* an aura attached to it because, at the time you make the choice of what to attach it to, Rancor is not on the field yet. You can't use Rancor as the "other" aura for Coronet to attach to something.