So I make Erebus in below Overworld and also tries it Below the Betweenland, but the thing is, there's a big gap between it and it's unavoidable death.

Minecraft-1.12.2
VerticallyStackedDimensions-latest
CD4017BE_lib-latest
Forge-14.23.5.2847

-BOP?

So it seems even including the following line in the INIT() of the .rcp file doesn't fix the transition portal layer spawning at y=256.

dim_ceiling:66 = 127;

Any idea why this isn't working?

the top of the erebus is naturally at height 128, like the nether. In the config file you can change what level the ceiling portal spawns at