James Asher Vampire Series by Barbara Hambly. Edwardian vampire adventure/suspense/mystery stories. The main (nominated) characters are James and Lydia, who are a married couple who are adorably in love, and Don Simon Ysidro, who is one of the oldest vampires in Europe, and starts the first book by blackmailing them into hunting down someone who is murdering vampires. But then they? Start to respect each other? Simon spends a week sitting next to James' sickbed reciting Shakepeare to him in the original dialect to keep him company and keep him safe from other vampires? In book 2 Lydia and Simon have to team up to save James and by the end of it they have confessed their love (which they shall never consummate, because they both care too much for James, and also Simon is a vampire who murders people to live and can't have sex) to each other and, separately, have confessed their love for each other and their resolution never to act on it, to James.

Eight books later, of escalating mutual rescues, hurt/comfort, and angst over the tragedy of vampires and humans who trust each other because of who they are and can't trust each other because of what they are, and even after multiple canon gay characters and a whole subplot where James and Lydia learn it's actually fairly common for a vampire to fall for a human couple, and it still hasn't occurred to James that it isn't actually normal for a monogamous man to be happy to let his wife go on adventures alone with the other man she's in love with, and he's the only one of them who hasn't realized yet that he loves Simon too and Simon loves him back.

But, like, I'm holding out hope, her other series has an adorable canon poly V in it, someday they will get there.