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.