Did vampires ever fight for Voldemort?

Question

As seen in this question vampires are mentioned several times in the Harry Potter books.

1. Hagrid mentions Professor Quirrell met vampires in the Black Forest.
2. One of Lockhart's books is titled Voyages with Vampires.
3. Lupin assigns an essay on vampires.
4. Slughorn invites vampires to his party where Harry meets one.

From these points, it is understandable that the vampires exist in the Harry Potter universe, wizards have good knowledge about them and some of the wizards have even befriended them.

When Voldemort rose to power for the second time he allied with giants, werewolves, Dementors and various other creatures. However, I could not find any mention of vampires allying with him. Did he get any vampires to fight for him?

Answer 1

Vampires played no meaningful part, implying they didn’t aid him.

While it’s theoretically possible that the Dark Lord had vampire allies that were never seen, as there’s no explicit statement that no vampires worked with the Dark Lord, the evidence is against it. Though Rowling’s never explicitly said that no vampires worked with the Dark Lord, she does state in her writing on Pottermore about vampires that they played no meaningful part in the story.

Although vampires exist in the world of Harry Potter, as shown by the literature that Harry and his friends study in Defence Against the Dark Arts, they play no meaningful part in the story.
_{^{- Vampires (Pottermore)}}

Since presumably working for or being allies of the Dark Lord would be considered a meaningful part in the story, this very likely means that no vampires were working with the Dark Lord.

Vampires as a species mightn’t be inclined to join the Dark Lord.

The creatures that join the Dark Lord tend to be evil, shunned by wizard society, or a bit of both. Vampires aren’t shown as being particularly evil or being shunned as a whole by wizard society (like werewolves are) so they probably wouldn’t be particularly interested in joining the Dark Lord. Neither of the vampires mentioned seem to be evil or particularly bad. Sanguini was a friend of Slughorn’s old student, and Slughorn trusted him enough to bring him to a party.

“Worple, who was a small, bespectacled man, grabbed Harry’s hand and shook it enthusiastically; the vampire Sanguini, who was tall and emaciated with dark shadows under his eyes, merely nodded. He looked rather bored. A gaggle of girls was standing close to him, looking curious and excited.”
_{^{- Harry Potter and the Half-Blood Prince, Chapter 15 (The Unbreakable Vow)}}

He does eye some humans hungrily, but he’s easily distracted by a pasty. Though Trocar never actually showed up in the books, he was going to be a teacher at Hogwarts whose subject wasn’t decided (meaning it wasn’t for sure Defense Against the Dark Arts, the only subject with a few evil teachers), and Rowling only mentioned him as being a teacher, so most likely he wasn’t intended to be evil either. Vampires don’t seem to be shunned either - people seem excited to see Sanguini. Notably, Honeydukes seems to sell candy for vampires, indicating they’re not particularly shunned.

“Ron and Hermione were standing underneath it, examining a tray of blood-flavoured lollipops. Harry sneaked up behind them. ‘Urgh, no, Harry won’t want one of those, they’re for vampires, I expect,’ Hermione was saying.”
_{^{- Harry Potter and the Prisoner of Azkaban, Chapter 10 (The Marauder’s Map)}}

Honeydukes, a ‘normal’ wizarding candy store, being willing to serve them shows that vampires are accepted or likely at least tolerated by wizarding society, and since they don’t seem to be evil, it’s unlikely that they’d have much reason to want to join the Dark Lord.

No vampires are ever mentioned as fighting for the Dark Lord.

There are several creatures who are mentioned as either the Dark Lord trying to recruit them, or as fighting for him. However, there’s no mention of him or his Death Eaters ever pursuing or having vampires on their side, despite alliances with other creatures being explicitly mentioned. The Dark Lord says he expects the Dementors and giants to join him, and he’ll have an army of creatures.

“The Dementors will join us … they are our natural allies … we will recall the banished giants … I shall have all my devoted servants returned to me, and an army of creatures whom all fear …”
_{^{- Harry Potter and the Goblet of Fire, Chapter 33 (The Death Eaters)}}

Both the giants (except Grawp) and the Dementors did join him, as shown later, and they weren’t the only ones. Greyback, a werewolf, was working for the Dark Lord. In addition to his using brute force to fight for the Dark Lord, Greyback was also helping him get other werewolves to join him.

“And this is the man Voldemort is using to marshal the werewolves. I cannot pretend that my particular brand of reasoned argument is making much headway against Greyback’s insistence that we werewolves deserve blood, that we ought to revenge ourselves on normal people.”
_{^{- Harry Potter and the Half-Blood Prince, Chapter 16 (A Very Frosty Christmas)}}

However, there’s no mention whatsoever of either the Dark Lord or the Death Eaters on his behalf trying to recruit vampires, or of any vampires who already joined him. It seems likely that if he had or even had tried to get vampire allies, it would have been mentioned at least once. No vampires come to fight for him at Hogwarts, though giants, Dementors, and at least one werewolf do. As that was his most important battle, it seems likely he’d want every creature allied with him to fight for him there, so that no vampires were at Hogwarts then implies that he didn’t have any.

Answer 2

It's doubtful. Since there are no quotes that explicitly say no vampires ever worked for Voldemort there is still a chance though it is slim. Here are a few quotes from Pottermore telling us about vampires,

Although vampires exist in the world of Harry Potter, as shown by the literature that Harry and his friends study in Defence Against the Dark Arts, they play no meaningful part in the story. The vampire myth is so rich, and has been exploited so many times in literature and on film, that I felt there was little I could add to the tradition. In any case, vampires are a tradition of Eastern Europe, and in general I tried to draw from British mythology and folklore when creating adversaries for Harry. Aside from passing mentions, therefore, the only vampire whom Harry meets in the books is Sanguini in Half-Blood Prince, who makes a faintly comic appearance at a party.
_{^{-Vampires - Pottermore}}

In this quote we cans see that the only (that isn't only briefly mentioned) vampire in the Harry Potter series is Sanguini.

Looking back through my earliest notebooks, however, I found that on my very earliest list of staff, there was a subjectless vampire teacher I had forgotten, called ‘Trocar’. A Trocar is sharply pointed shaft inserted into arteries or cavities to extract bodily fluids, so I think it a rather good name for a vampire. Evidently I did not think much of him as a character, though, because he disappears fairly early on in my notes.
_{^{-Vampires - Pottermore}}

So there is one other vampire that was removed from the series but he didn't work for Voldemort.

For a long time there was a persistent fan rumour that Snape might be a vampire. While it is true that he has an unhealthy pallor, and is sometimes described as looking like a large bat in his long black cloak, he never actually turns into a bat, we meet him outside the castle by daylight, and no corpses with puncture marks in their necks ever turn up at Hogwarts. In short, Snape is not a revamped Trocar.
_{^{-Vampires - Pottermore}}

If there are any suspicions about Snape, they are untrue. The only creatures we know Voldemort recrutied are the Giants and the Dementors

“The Dementors will join us … they are our natural allies … we will recall the banished giants … I shall have all my devoted servants returned to me, and an army of creatures whom all fear …”
_{^{- Harry Potter and the Goblet of Fire, Chapter 33 (The Death Eaters)}}

So in conclusion there are no vampires in Voldemort's employment that we know of.