Boy you can't just mention something about rot and decay vergil headcanons and not elaborate
aight so I have like 2am post about this here but I like it so I'll give you the brief overview
so there is a god of rot and decay trapped under Vergil's skin (like Malenia, I take a lot if inspiration from ER) and he's just been dealing with it (tm) for his entire life, but it's not apparent because of his demonic healing. He was a sickly child because of it though cause he hadn't unlocked that healing entirely yet.
so he's been keeping this mf at bay through demonic healing and willpower and doesn't want to be a god because he'll gain power but think's he'll completely lose himself in the process
however, I can see a scenario where he does ascend to protect his family and is not coping well with the new changes
(also he may or may not be a vector for the wasting sickness and this has totally not killed a past love leaving him heartbroken and afraid to connect with anyone)
(and the cut roots and branches of the qliphoth may or may not be rotting with this sickness, causing a horrible pandemic in the demon world which definitely won't make it's way into the human world)