First, since vampires are bestselling, I'll have a hot vampiric seducer. Since male vampires have overstocked the market, I'll venture out and put in a female vampire, since they're so mysterious.
Mary Sues being in fashion, the hero is a good-natured inexperienced young man. He is also a super-intelligent graduate. His bride is a pure, innocent, sweet being who dotes on him. A triangle between the hero, the young bride and the vamp ensues ... though not in the way you expect.
Since revenge always adds drama, the female vampire wants revenge on men for some undefinable reason. She decides to wreck this through the agency of the hero by seducing him. Gratuitious seduction scenes are optional - perhaps even encouraged.
But you mustn't forget the Byronic heroes, lest you alienate the countless score of fangirls. The sheer number of idiots who love Twilight almost makes me wish we were back in the dark ages when people couldn't read or write. These people DESERVE to be illiterate. The hero is a brooding young man with some undefinable existential crisis, because ... he's not allowed to be with his bride, whom he has just eloped with. That's it!
Also, since all bestselling erotica attempts to be intellectual be referencing the Bronte sisters, Thomas Hardy and the sillier aspects of Shakespeare's plays, let's do the same. For example, in my version I'm putting in La Belle Dame Sans Merci from Keats and Geraldine from Coleridge's Christabel. Since Romantic poetry looks waaay more cultured than romantic novels, or rather classic novels stripped of everything except the romance bits. To mix it all up, I'm doing a Melmoth the Wanderer, which is so gothick.
So what do you get? A mash-up of genres, including fantasy, suspense, romance (this is for the most stupid of the readers, and most readers are stupid, as literacy levels prove) and horror, Scifi would be cool, unfortunately that's for the guys. Sorry, guys, but you don't make a majority of the readers who send books up bestselling lists.
So how does the vampire seduce the hero? Since he's supposed to be incorruptibly pure, she disguises herself as his bride. This device, by the way, was used in Shakespeare, in case you're wondering. Yet another way to put in something stupid and sound intellectual. The bed trick involves Mariana pretending to be Isabella in the bed of the judge - somethnig subsequently used in Restoration dramas. The Restoration by the way was full of this sort of crap which is why they don't last today - or perhaps why they've recently made a comeback. since we are now all over-sexualised morons. But let's go a step further from the bed trick at night - and put it in a Doppelganger element, because that's just so goth. (It was in the 18th and 19th century). The fairy changes form (which is supposed to be in Coleridge's Christabel only he never got round to finishing it) and transforms into the hero's young bride. Then she gets to seduce him, preferably with a song like the sirens. Let it take place in a small pool outdoors, because sirens are associated with the sea.
It is frightening to think that one can write a story just out of a bunch of stupid cliches and tropes. But add the atmosphere in and you're done. Now that is something else entirely.