Way to create drama on your birthday.
Guys complain that they hate girls that bring drama... yet you do things like this? Not always our fault.
The one your friend is into is a NO GO.
Your Ex. Well, you're an idiot cause she's just using you for a screw now and then. That isn't going to happen, get over it. You're probably going to piss her off a bit by hooking up with another girl, but we're jealous like that so whatever.
You're left with the slut and the one your friend isn't into (forgot if that is the 25 or 75). Anyway, sluts have feelings to. If you're about to go on vacation with this girl and share a room, you don't want to piss her off. You are going to end up with stupid drama at your birthday and even more lame drama after if you fuck this situation up. I think you realize this.
Also, just because a girl is massively flirty, doesn't mean she is down to fuck. How to know if she wants to fuck: She tells you, she has told you about her underwear, she looks at you with eyes that pierce your soul, or you've already fucked her. If none of the above happen and she's just really super flirty and you know she's been with other dudes, she isn't down to fuck, she's down to have you chase after her and maybe take her on a date.
It's your birthday, go the path of least resistance. The slut is the way to go. You know you can get some and will cause no drama for the week after. If the other girls are truly down, you'll make em a little jealous and they'll just try harder next time. As far as the ex goes... get over it.