Well, personally, I wouldn't differentiate between girls/boys so much as to look at the personality of whomever he is facing. I don't think gender matters to Vash as much as how annoying the other person is.
He has a thing for the weak and feels the urge to protect them (liechtenstein, austria...), regardless of their gender. For the rest, he will either be indifferent or annoyed, depending on how the other person interacts with him. For example, he shoots France during the christmas special when he takes a picture of him in his pajamas. I can TOTALLY see him shooting Hungary for the exact same reasons, it wouldn't matter that she's a girl. I'd probably look into the history, as well, as Vash seems to hold his grudges for a LONG time (see Austria). Here again, I don't think he'd make a difference between male or female.(and then, I see him bickering about chocolate with Belgium A LOT, but it's nothing serious, it's just a thing they've done for ages and will probably never stop doing).
Re: Help portray Vash?
He has a thing for the weak and feels the urge to protect them (liechtenstein, austria...), regardless of their gender. For the rest, he will either be indifferent or annoyed, depending on how the other person interacts with him. For example, he shoots France during the christmas special when he takes a picture of him in his pajamas. I can TOTALLY see him shooting Hungary for the exact same reasons, it wouldn't matter that she's a girl.
I'd probably look into the history, as well, as Vash seems to hold his grudges for a LONG time (see Austria). Here again, I don't think he'd make a difference between male or female.(and then, I see him bickering about chocolate with Belgium A LOT, but it's nothing serious, it's just a thing they've done for ages and will probably never stop doing).
Uh. I hope that was clear?