Something for you to try. Create a new user folder. Just rename you existing "The Sims 4" folder to something else or move it to your desktop. Start the game so it creates a new one. Make the sim and place them then save and quit. Copy just the save folder from the previous user folder to the new one then start the game. Do not copy your Mods folder. Play Gunther a little bit and attempt to make him a vampire again through play. Don't use any cheats.
If it works and he becomes a vampire, then it's probably something in your mods folder that caused it which you have to hunt down and either update or remove. Once you find whatever it is, try again with the original save to see if it works and Gunther becomes a vampire. My guess is that you have a CAS CC item that you used on Gunther that is not tagged for use by vampires or broken but that is just a guess. It could be anything - custom skin tone, tattoo, clothes, jewelry, pretty much anything. You'll need to check everything.
Hope this helps.
I took all of my CAS CC out when I tried using the cheats and Mods to change him the first time and had the same problem, but I did what you said - with no mods - just in case. Thankfully, it definitely has nothing to do with my Mods folder. Gunther went through the process of becoming a vampire, the animation happened, and the Strange Thirst moodlet appeared, but he still had Sim needs. I took him into CAS as well, and as I thought, just a human form. No dark form at all.
This is also happening to me. I’m doing a Creepypasta vampire playthrough and Sally Williams won’t turn into a vampire. I used all the cheats I could find and it still wouldn’t work. I killed her beforehand and revived her by the wishing well. This also happened to Rainbow Factory and I had to reset her whole character. I’m on PS5 so I don’t have a mods folder.
