The only reason I suggested more generic names for character vampires is for players unfamiliar to the LOK series as a whole, but up to u.
ok, i'll try to come up with a preliminary design or build for a combined portal.
Human guardians respawning? I suppose that's ok.
I talked to a friend of mine who's a bit code savvy and does play minecraft (he doesn't like java so doesn't code, he could but chooses not to), this is what he said when i asked him for advice on this:
"The best I can suggest is it [the spectral realm] creates a dimension, same terrain generation, but also replaces blocks with impervious versions. Bedrock but same textures, maybe slightly different. I wouldn't suggest recreating the world block-by-block because that would eat up a ton of RAM, depending on how much of the overworld is already generated and built.
OR, when the player enters this world, it could activate a set of custom Game Rules, where the daylight cycle stops, certain mobs spawn, certain textures replaced, Water has no effect, and would put the player in Adventure mode (can't interact with blocks n such, but can interact with inventories and mobs.)"