''Again, just imo Vergil copulating with a human as a teenager is more believable than Sparda having a child with someone after Eva, OR having an unspoken sibling for that matter.''
But nobody said he had a child with somebody after Eva. Where are you getting this from?
Even if he did have a child with somebody other than Eva before he died, that would be very strange, and an assholish move towards Eva, since she was still alive and had just birthed Dante and Vergil. It would also mean Nero can't be only 19 while Dante is in his 30s in DMC 4.
I didn't say he had an unknown sibling either. That said, can you really completely discount the idea? Isn't it somehow possible that Sparda's brother/sister had a son (Nero) with a human too? Of course, one big problem with this theory is that Nero is far weaker than Dante, and he doesn't get his own Devil Trigger, which to me indicates he is not a half-demon, but maybe a quarter demon.
If he is indeed 1/4 demon, then he is either Vergil's son or the son of Sparda's sibling's half-demon child. Sure, Sparda's relationship with Eva was legendary, and his children too, but that was only because they were well-known. Sparda was the demonic ruler of the humans. Nobody would care much about his brother/sister, especially if they kept their child secret.
That said, if he is an experiment, he could be anything. I'm sure that in the DMC world, anything is possible - it all depends on DMC's writers. He could even be 1/8 demon if the writers wanted him to be.
In any case, I can't believe a 30th anniversary character encyclopedia if all it says is that ''fans/people think Nero is Vergil's son''. I mean, duh. We need confirmation, not a recap of people's assumptions.