Man, that's a wild combo that shouldn't work but somehow really does. Viking vampire stuff, when it's done right, isn't just about sticking fangs on a berserker. It's this collision of two brutal codes about what it means to last forever. For a Viking, honor and a 'good death' in battle get you into Valhalla—immortality is earned through glorious, finite sacrifice. For a vampire, immortality is a curse you're stuck with, watching everything you fought for turn to dust. So you get a character cursed with eternal existence but shackled to this warrior ethos that says your worth is measured in a final, mortal stand. The internal conflict writes itself.
I keep thinking about the short story in 'The Bloody Rose & Other Tales' where a Viking lord is turned right before a pivotal battle. He spends centuries trying to find a fight worthy of finally killing him, but he's too damn good, and the 'honor' in slaughtering mortals feels hollow. It twists the original concept of honor on its head—it's not about proving yourself to your peers or gods anymore, because they're all gone. It becomes this deeply personal, obsessive quest for a meaning that maybe doesn't exist anymore. The loneliness is the real enemy, not the sun or a stake.
These stories also play with the idea of legacy. A Viking wants songs sung about him; a vampire often wants to be forgotten. When you mash them together, you get this being who might have been a legend, but now has to hide his true nature, watching his own saga get warped or erased by history. There's a bitterness there, a rage against the eternal night that feels very different from, say, a broody Victorian vampire. It's more active, more violent. The struggle isn't to resist feeding; it's to resist the nihilism that comes with outliving your own worldview.