Vergil didn't forget. He instantly remembered what Dante must mean once it was brought up. He didn't have a reason to remember.
As to why he didn't have a reason to remember... There are many possible ways to resolve that one. Could've been a real romance but with no consequence to the present, could've been a fake romance, could've been a prostitute... I even saw one fun theory where having sex was a requirement of some magic ritual, and a kid was unintended result of that lol.
I'll use Dan's interpretation as a starting point (if only because I predicted exactly the thing he said a few months before I heard it) and explain my own favorite theory on this.
It was a real thing. I just can't see it any other way: Vergil is a type to take everything he does very seriously, and commit 100% into it. It wasn't just a prostitute hired for the hell of it, or a fleeting crush.
What I imagine could've happened is... love at first/second/third sight isn't very common IRL, but it does happen. And if someone like Vergil took an instant interest in a woman, she must've been something very special, something about the character of that woman rather than just appearance. I find it very likely if the woman was in some sense similar to Vergil's own character, but that can be played with.
So although it wasn't a long romance (could've easily been only a couple of months), it was very, very real, and progressed very, very quickly. But then he had to leave. Could be several reasons, really:
- He had his own quest to finish already, and didn't have the luxury of bogging himself down with relationships.
- He had his own quest to finish, and, in fact, this short romance reminded him exactly why he was on it. "And without strength, you cannot protect anything.
Let alone ones you love." (<--as Dan said on a convention)
- He had his own quest to finish, and he realized he is becoming too human. Probably the most likely outcome the devs will use if they went with this theory of mine, given his character interpretation at that point in timeline.
- Something might've happened on the woman's side too, ya know.
Regardless of what exactly happened, the relationship was done and over with. It was important to Vergil at a time, and very likely it's still a memory he cherishes, it just had no consequence to the present... until that consequence came in and kicked his deadbeat dad ass.
Brb need to replay M20 again