If it helps any of the old guard to understand it better, here's how I'd classify each of the universes:
The Messageverse: The universe of the MBL and TOMB, and all their auxiliary teams and characters.
The Vanguardverse: The universe of Vanguard, Strikeforce, and all their auxiliary teams and characters.
The Metaverse: A universe that combines the histories of the Messageverse and the Vanguardverse (minus a lot of baggage, such as anything that alters history too much from the real world's timeline), plus a little JLRverse thrown in for good measure. The Metaverse is basically a background, a canvas upon which this story can be told, but while this story is told in detail, the background is painted with a broad brush.
