As far as we are shown on the show all the vampires with bigger roles are predominantly heterosexual, with small hints that they might be willing to experiment, like Drusilla and Darla doing the Immortal together, or that one time of intimacy that went down between Spike and Angel.
Though I have no qualms to slash anything that moves (I'm without conscience that way :) ) it comes extremely easy to slash Spike and Angel, in fact I often feel that canon is missing a piece without Angelus and Spike having a slashy past. When I try to nail where that notion comes from, I get to these notions:
1. There's the obvious subtext between Spike and Angel (what with all the boy-calling in S2) and Darla and Drusilla.
2. I don't like the idea that becomming a vampire magically changes your sexual orientation to bisexual (though vamp!Willow kinda fits that theory more nicely than she does fit mine), but it does take away your inhibitions and it gives you lots and lots of sexually active time. So it becomes very likely that a person that has even the slightest inclination towards her own sex is going to act on it at one point or another. There are gender theories that say, we are all bixsexual, just with variable percentages and if that's the case I'd say, vampires are the most likely creatures to explore every corner of their sexuality.
3. This last thing is a rather messed up theory about how the changing morals of their times influence a vampire, more specifically Angel and vampires roughly his age and upbringing. There were large chunks of christian dominated history where homosexuality was considered evil. To the point where homosexuality was not only a sin but also a crime that could get you burned, same as being a witch or a murderer. Angelus is obsessed with the catholic church, which condems homosexuality to the day but had much more influence back then. He loves to snack on nuns and is pretty much set on committing every sin they ever named.
That homosexuality being evil is load of bull is self explainatory I hope, but it would not be for a 18th century man who grew up with a clerical education and believe system. Homosexuality would have been presented to him as evil all his life from the church as well as from the law.
So where does that leave the vampires, who identify as evil, even are by their nature evil. I'd say in general it doesn't affect them much since in the Buffyverse evil is not subjective ( like it is in reality), so I'd take it since being gay is not really evil, demons are not automatically bisexual. But if someone is philosophically obsessing about reaching the maximum of evil, the way Angelus is, he would take every moral taboo of his time into consideration and would strive to break it. Angel and Angelus are both perfectionists, I can see both of them with their little lists of sins, Angel regretting and Angelus not wanting to leave any out. So to me it's a given that Angelus did it with men just because in his frame of mind that would give him more evil points.
How Angel deals with that would be interesting. Times have changed, Angel changed and he's not a homophobe. But he'd know Angelus did men for the love of evil, so that would be bound to give him major hang ups about it.
Spike doesn't have the same issues, since he was never that obsessive about being evil and doesn't make so much of a difference between the souled and unsouled version of himself.