I would have guessed Dark x Sweet too but since I know that it's OkameP, I know that he probably uses V2, unless it's otherwise specified. Since he credited a Miku Append in some songs but not in all, I guess that this one is a chorus of Dark and Original/Sweet. I just have no clue at which octaves they sing.
I experimented with XSY too and liked Dark x Sweet but they didn't sound actually close to this. Maybe I should experiment with the parameters a little or I just have had a bad contrast between the octaves and pitch bends. (Like I use lower octaves and higher pitches etc., you know?)
EDIT: BTW, I'm not working on an English song at all ATM. It's Japanese, so you don't have to think about Miku English.