For some reason whatever Illusion spell I use the skill in general doesn't level up. I had it at level 100 once but made it legendary and it got reset to level 15 (as it should) but the text is in red, set as if it was level 13 eventhough I don't have any decease on my character. She's a vampire so she cannot even become sick.

What is going on here? I can use all spells but I seem to be stuck at level 13/15. :'(

Disease*

Must be a bug, not sure why that might be. If you are on PC, you can fix these issues with the console, if not, then aside from reverting to an earlier save, not much you can do about it. Trying praying at an altar, occasionally diseases won't be shown yet they'll still have a negative impact on the player. In addition to that, there is also a bug that sometimes still lets the player become infected with a disease from traps even with 100% disease resist from Beast Blood or Vampirism. There are conditions to leveling every skill in the game (ie you need to sneak near a creature/npc/etc, sneaking alone doesn't give exp), you may not be meeting the conditions to leveling up Illusion.

It shows level 13?

Cure Vampirism and/or Use a shrine of Tacos or any other gods

Do they even have tacos in TESland?

Ask Heimskr

I recommend searching "Gamer Poop Skyrim #5" on youtube to find that answer.

I've ritten myself of vampirism as a start and suddenly the Illusion tree is back to its normal 15, but it still won't level. Praying doesn't help. It seems no Talos (or Tacos) can help me in this, so I have to went myself again to the alknowing members of this forum.
Is there something else I could do?

I am on pc btw.

Post all mods you have downloaded. Another PC user could probably help you then.