It's just one thing I wondered about. Vampires seem to have pale hair(blonde, or somewhere between silver and light blue), light skin, etc. and often red eyes. Sakuya is often depicted with red eyes, is pale and has light hair(silver, sometimes tinted blue).
I really like the theory that she was a vampire hunter. Her haircut really looks like a "strong female warrior" haircut(maybe I'm just saying this because it looks like Reina's from Queen's blade) just with bows on it.
She seems to willingly go along with her role for some reason, and it seems odd she'd do that unless she went all Lord Genome or something.
I think maybe what happened is that Remilia at the time couldn't outright defeat her, and instead infected her with Vampirism. As long as she stays with Remilia, she would manipulate Sakuya's fate so it would not develop. This is why she can go out during the day, she only has the initial cosmetic onset of vampirism, she has the light haired, red eyed rather unsettling look but has not developed any kind of allergy to sunlight.
In the games you play as Sakuya in, it seems that nature means a lot to Sakuya, it was of her own interest as to why spring was not coming, etc. so it would be understandable she would not want to loose the ability to freely move around at full strength during the day. As long as she remains with Remilia, she does not fully develop vampirism and can do this.
Either that, or it was Flandre that infected her, Sakuya was sent to kill Flandre since she's more of an actual threat, and Remilia in some smug arrogant way took pity on her.