Gemma Arterton says she signed up to do the film because she was interested in the mother-daughter relationship and because of its feminist themes. The producer remarks that vampire movies are rarely if ever led by two female protagonists, which makes this interesting in itself. But while Arterton and Saoirse Ronan have some room to dig down into their characters (the two have opposing sensibilities: one loudly untroubled by the memory of the past and the other quietly obsessed with it), the tension between them boils down to the necessity for the child to fly the nest and be free to "tell her own story".
So where does the feminism fit in? Firstly, in its unique twist on the vampire mythos. These 'soucriants' have an interesting variation on the 'vamp face': their thumb grows a talon when they are about to kill, and also unintentionally when they are sexually aroused. This is a more "phallic" weapon than a mouth ringed with fangs, and in claiming these metaphorical dicks for themselves, the vampire ladies can be seen to challenge the supremacy of the male vampire.
Indeed, the plot of the film revolves around a challenge to male authority. We learn that Arterton and Ronan are in breech of a code (enforced with maximum prejudice) by a cabal of vampires who insist that only men can be trusted with immortality. The fact that this breech occurs 200 years ago with the French Revolution and the Rights of Women may not be an accident. They are on the run, and set up shop in a seaside town where Arterton takes over a disused hotel called Byzantium and converts it into a brothel. The brothel is used in part to contrast with the safe (sex-free) spaces of the orphanage where Ronan grows up and the school she goes to. But its name takes on a new significance when it is revealed that the vampire patriarch is an old Greek who fought in the crusades. His 'Byzantine' attitude is what the women are up against: medieval, outdated and showing an undue reverence for ancient and obscure rituals which have been robbed of meaning by the march of history.
It's Buffy vs the Watchers Council, basically, and the comparison gets at something interesting about where the film ends up. Neil Jordan had worked with Angela Carter when adapting her short story collection The Bloody Chamber into the film A Company of Wolves, and I wonder how far that influence carries over into this film (Ronan wears a conspicuously red hood when delivering death to old grannies). I say that because there is a faint echo of the Bluebeard story in the climactic final sequence of the film, in which Arterton is bound and about to be beheaded by the patriarch. In Carter's retelling, the build-up to the seemingly inevitable execution of the heroine is disrupted in the final moment by an out-of-the-blue intervention of a female maternal force. Something very similar happens at the end of Byzantium, but in this case the force is male and romantic.
This potentially troubling conclusion is in part defused by the fact that we see Arterton behead a male vampire at the beginning of the film, and the fact that in many respects she's a very Carterian heroine (I think Arterton would be perfect playing Fevvers from Nights at the Circus). So perhaps there's a way to rehabilitate Sam Riley's role in rescuing the damselled Arterton. While Arterton has the willpower to survive in a patriarchal world, this inevitably leads to a lonely existence in which she is subjected to and subverts the desires of men. Riley holds out the possibility for another kind of relationship amongst equals. But in order for this victory to be achieved, Riley has to betray the organisation that empowered him and denies him a romantic life. Dismantling patriarchy, the film suggests, requires many more such defections.