I really can't vote. With what they've been through and given their circumstances I don't think either of the twins are really the girl next door type, or any other ordinary girl for that matter. Maybe for a short term relationship, or a one night stand, but if they took a girl as a steady mate, I think she would definately have to stand out, be someone they could relate to, and/or be very different.
That's probably why so many people pair Lady with the twins. (That and the fact that she's pretty much the only woman in the series isn't trying to kill them, related to them, or already taken. XD) She is not only a demon slayer but her circumstances are something the twins can relate to.
If Vergil chose to live among humans again I think a regular gf is out of the question, maybe another female demon slayer or a woman who has had unfortunate run ins with demons as well, but strong minded, someone to compliment or balance out Vergil's own emotions.