Vega is no where near the level of any of the Darkstalkers. In the Darkstalker world, all humans are given a Rating, (Let’s say B for example, the exact terms escape me) and are not on the level of high level Darkstalkers. Especially one such as Anita who is one of the strongest Darkstalkers at that. Like I said, Vega is not 100% Human Being, but he’s still not on Darkstalker level.
Think of it this way. What is the worst thing someone from the Street Fighter World can do against, say, Demitri. Perform the Shun Goku Satsu to send him to Hell for one second? HE’S FROM THE MAKAI WHICH IS A HELL DIMENSION. Instead of the Demons attacking him, they’ll probably slap him five. Darkstalkers own SFers for free.
Me, personally I tend to feel that Vega and Rose were one, and when they seperated, the Rose that appeared (On her birthday) looked fairly like our Rose. If it happened another way, you’d have to explain Vega tracking down this baby to train it, and he did train Rose.
While such a concept of one person splitting in two maybe a strange concept to some, it is very common in Japanese Movies, Anime, and Manga. Quicky examples -
Dragon Ball Z - Piccolo pretty much did the same exact thing.
Sadako of the Ring Movies - One person split into two. One Sadako grew up to lead a regular Teenage life, the other remained frozen at a young age and Evil at that. Later the two combined into one.
Berg Katse of Gachaman (AKA G-Force, Battle of the Planets, etc. Leader of Galactor.) Sosai X took two twins in a mother’s womb and combined them to give BK a high IQ. Sometimes he was Male, sometimes he’d morph into female.
X (1999) - The Anime not only reveals that Kamui and Fuma are the same person because of fate, they later combine into one.
There are other examples, but these are the ones that pop into my head at the moment.