Vega is able to enter The Catacombs through a keyhole within his personal diary with a key that only he has. When unlocked, access is granted to his own personal dimension, which starts off as a giant cloud of swirling information. If a normal person were to dive into it, they would be lost inside the wave of memories and events and explicit information until they eventually become fictional. Only The Blind Man is able to walk through it intact.
Inside of this cloud of information, is an island fabricated from the universe's fabric. On this island is a structure that varies according to which version of Vega appears in front of it. If the Old Vega appears in front of it, it manifests as a giant stone structure filled with a vast amount of the Blind Man's most prized information. If the Young Vega appears, then it manifests as a little shack with a small bookcase full of parchment. Inside of this structure, is something that leads downward; either a spiral staircase, a trapdoor, or whatever represents the version of Vega currently using it. Once he descends into this area, it leads into the Catacombs, an underground tunnel with passages where thousands of books are kept with a spiral spider web at the very bottom center. At the bottom of this web is one book that contains the past memories of the days before The Dying Man was destroyed and what creatures lurked on the earth back then, and his pet spiders Past, Present, and Future.
There's a tiny hole to existence in the Catacombs as well, and it's where Vega became the "Blind Man". Through this hole, he can see The Quiet slowly eating away at the boundaries of the universe.