I assume you want to do this in addition to the Crimson Curse? Beacuse if not, a simple way would be to make your disease, assign it the vampire tag, and replace the existing Profile Icon and Disease Panel Overlay (i.e. name them the same thing).

I've been diving around the files and no luck on how the effect is actually created. I would have thought the DLC created a new character.layout.darkest file, but it doesn't seem like it. It seems to be hardcoded.

You may be able to use the system even if it is hardcoded though. I would attempt what I proposed above, just to see if it works. After that I would recommend creating your new disease, and assigning it it's own custom tag. Then name your Disease Panel file disease_overlay_yourtaghere.png and put it in the shared/character directory of your mod folder. Then same with the Portrait Icon: name it portrait_icon_yourtagehere.png and put it in the shared/hero directory. That would be my best guess.