You must gain these skulls:
Assembly Skull: Walkthrough coming soon. Mythic Map-pack skull.
Orbital Skull: Ditto. Mythic Map-pack skull.
Sandbox Skull: Ditto. Mythic Map-pack skull.
Longshore Skull: Ditto. Halo 3: Mythic map skull. Halo 3: Mythic is the multiplayer disc for Halo 3: ODST -- it contains the Mythic map-pack and three new maps -- these maps' skulls will net you the Vidmaster.
Citadel Skull: Ditto. Ditto.
Can't remember other map. Ditto. I'll add more later.