Yes, they can get erect.
Ok, even though we've talked about this extensively, I wonder why, there are some new thoughts I have that may seem interesting. First of all, if we are going to go by the lore and by the most drama potential, we have to say that the physical side of sex means nothing and it doesn't feel different than being touched on any other part of your body. This is mostly a curse, not a better form of being a human. When we remove the biological drive of having sex, but also taking into account what it does for the vampire outside of strengthening social relationships, what is left is the purely intellectual desire for some kind of intimacy that they try to fulfill with sex. The vampires are hollow husks in terms of emotions, they don't feel anything apart from the hunger for blood, the instinct of self-preservation, and some form of orgasm when they bite someone, so the act of sex itself is purely a mechanical exercise, no more pleasurable than holding hands. This can be great drama potential, the vampire that longs for the thrills s/he is now denied, like Don Giovanni, yearning for love that doesn't exist. Or a way to explore different kinds of intimate relationships that don't rely on sex or emotions.
The lore has a kind of workaround - the Blood Bonds. The only way for a vampire to feel just a slight echo of emotional intimacy, it isn't like being a human, but it's through the only thing vampires have, blood. It's infinitely faint, though, it's not "just as good as the real thing" and that's also great drama fodder. I don't know why I feel the need to repeat that this is and should be a curse. This is why vampires become alien in their mindsets after a while, they completely forget what it was like to feel anything outside of hunger.
