The Volcanic Vault is an exclusive toy, part of a special sales action at Game (UK) or Best Buy (US), where it was included with the Wii/360/PS3 Starter Pack. When the toy is placed on the Portal of Power for the first time, it unlocks the multiplayer Volcanic Vault arena for play in Battle Mode.
The Volcanic Vault arena multiplayer level is a recolor of the Aqueduct. It's exactly the same except for the colors and the fact that it uses lava instead of water.
In Skylanders: Swap Force, instead of opening a new arena, a rain of fireballs will appear, damaging nearby enemies when placed on the portal.