Volcanic Vault is an exclusive battle arena in Skylanders: Spyro's Adventure. It was available exclusively at Best Buy in the US and at GAME and Gamestation in the UK. The US release came without a case where as the UK release did. To unlock it, simply place it on the portal as you would with a Skylander. This item only works with the console versions only.
Volcanic Vault is a lava version of Aqueduct arena. It functions the same, but looks different.