What counts as a spoiler depends on your table...
The best source for lore on monsters for DnD 5e is the aptly-named Monster Manual book. It contains varyingly long lore dumps ranging from a couple of paragraphs to short stories. The descriptions are often quite interesting! Each is also accompanied by the stat block of the monster.
Since you're playing Curse of Strahd, the module's book itself will contain lots of background information that the GM may or may not allow you to view. It is noteworthy that the Monster Manual contains a backstory block for Strahd himself, so there is definitely some minor campaign spoilers there too. Ask your GM to be sure.
Outside the particular scope of vampires, other DnD 5e books also include monster or location lore as a part of their content. You can look up books like Mordenkainen's Tome of Foes or Volo's Guide to Monsters to get access to more and deeper lore dumps.
Whether the lore or the stat block are "open information" varies between different tables. Our table has usually run under the assumption that most of the lore presented in the book is reasonably common knowledge that has been passed on to our heroes through folklore, their earlier careers or other experience. Stat blocks are usually considered to be spoilers, but your mileage may vary.
...so ask your GM to meet you half-way
Ultimately, it's the wisest to ask your GM to point you to the lore they intend to use. This not only ensures that the GM can only provide you with the information that doesn't spoil the campaign, but also ensures that the information given is not misleading in the event the GM decides to deviate from the more established sources of monster lore.
Finally, given the nature of the campaign, I'd avoid playing an experienced vampire hunter without strong and explicit approval of the rest of the group. The gothic horror atmosphere of the campaign is supposed to flood the players with grim mystery, that might be diluted were your character to claim a specialized knowledge in the workings of vampires.