Hi, I don't think it's fair to write this off as vagueness or laziness on Hino's part. This is a hallmark of East Asian storytelling, where a lot of things simply don't need to be spelled out, and that's tied to the tradition of indirect expression in kanji-using cultures. It's also why so much of the mood/subtext doesn't survive translation into English.
What I'm trying to get at is that VK's worldbuilding, and its central doomed romance, draws straight from Vampire Hunter D. It's a story where a half-vampire, half-human hunter belongs nowhere, crushed by the isolation of immortality, rejected by humans and never at peace with the vampire side of himself either. Kaname isn't a dhampir, but he's the one who founded the anti-vampire weapons in the first place, and his situation takes the same shape: a pureblood who's been around for centuries and never come to terms with his own identity. To pass himself off as human and fit into human society, he never let himself bond with anyone, and over time he forgot his own name, because no one was ever there to say it. He refused to feed with his fangs, but couldn't outrun what he was, ended up back with the race he despised.
And this isn't just me reading too much into it either. In a 2008 interview [1], Hino herself flat-out recommended this exact novel, describing it as "living precariously, teetering on the knife's edge between human and beast." That line pretty much sums up Yuuki, Kaname, Zero, and even Kaien in one go, none of them fully human, none of them able to fully own their vampire side either, all four caught between two worlds and never quite landing on either side.
Kaname mirrors the strongest Vampire in Vampire Hunter D pretty closely too. Hino writes his deep weariness toward an endless lifespan, and his fixation on turning Yuuki human is arrogant, basically the Vampire's line to his beloved: "You don't understand the pain of having to live forever, I don't want you to experience that pain," met with his beloveds response, "That's only what you believe,"[2] which is close to what Yuuki throws back at Kaname. Yuuki had already decided to follow him anywhere, even to hell. In the end of this tv series it's implied the woman dies and the vampire follows her in death. Whether they actually die is never explicitly confirmed, I think that's just how East Asian storytelling rolls.
This also lines up with what Hino's said about VK's origins, her initial concept was an extremely tragic short story, not a school shoujo romance. Her editor at LaLa rejected that pitch and pushed for a school setting with a bunch of hot guys instead, leading to an opening arc that reads like such a generic school romance and pulled in a fanbase that wasn't really there for a dark story, so it backfired on her later.[3]
So no, I don't think this is Hino not knowing what she was doing. She was clearly trying to draw from the Vampire Hunter D concept of an extreme tragic romance, even within the constraints the school setting imposed on her. Yume was arguably doomed to a bad ending from the very start, and the actual ending of VK Memories is Hino's mercy.
Lastly, and to be clear, the narrative logic never required him to have a wife, just a bloodline heir, and that's why the manga keeps hammering words like "breeding" and "beast". VK constantly reminds you that purebloods don't pair off for love, they pair off to keep the bloodline going. Rido has a child and never married. (and the manga also makes Rido a mirror for both Kaname and Yuuki) So why not just make Kaname the uncle, with the Kuran bloodline coming from his brother, because Hino was writing incest on purpose, an ancestor falling for his own descendant, a gothic, dark relationship. There's even a line in the manga that says she inherited his most concentrated blood, that's the effect Hino was going for. Without that taboo, the whole story loses its point.
[1][3] ぱふ 2008年 08月号 雑誌 "巻頭特集: 魅惑のヴァンパイア 「ヴァンパイア騎士」
In the year 12,090 AD, the world has become the domain of vampires known as the "Nobility." Though showing signs of racial decadence and decline, the Nobility still threaten humanity with their overwhelming power. Among the hunters who specialize in hunting them is a young man renowned for his exceptional skill and striking beauty. His name is "D." He is a Dunpeal — born between a vampire and a human… The series has also been adapted into other media, including OVA anime films, and remains a hugely popular series by Hideyuki Kikuchi.
[2] Vampire Hunter D: Bloodlust (2000)