The Rim is a solid wall of steam that connects The Umbilical to Sanctum

cutaway image of the Rim from one side to the other.

The vines are the only thing that can survive in the Rim, it even erodes mountains over time.

Outside of the Rim is the NomadLands, and, Vines become much less dense outside of the Rim

The Veil itself never actually touches the planets surface, the umbilical brushes very close but stops several miles from the the crust, Water vapors from the Veil get locked in this position and the Rim is formed between the Surface and the Umbilical. The Rim itself is highly electrically charged steam, able to burn the skin off any creature who touches it.

It was like a cage that contained the Bound inside the Vinefields and it was thought to be unpassable until The Passage was found. A mass of Vines created almost a perfect tunnel from on side of the Rim to the other.

Two Bound ontop of the Vinefields travel towards the Rim