There are more ways to balance a battle than just nerfing monsters.
Quick Side Note: the following suggestions have the same final result as weakening the vampire, even though we aren't changing the vampire's stats. You might find you want to weaken its stats and use some of the following approaches - a combination of the multiple ideas could be powerful. However, if all that is desired is a starving vampire with a decreased CR, go with some of the other excellent answers that have some wonderful ideas. It has been my experience though that CR rating is more art than science.
One quick illustration: Fighting a swarm of bats may be trivial. Fighting the same swarm on a rickety wooden bridge (which means constant balance checks, and no maneuverability and single-file) over a pit of Lava (lethal consequences for failure) makes the Encounter vastly more difficult, even though the CR of the Monster hasn't changed at all.
There are numerous ways to make the Encounter with a vampire easier, without changing the stats for the vampire itself:
- Special, specific game-changing rewards given before the finale. These can be weapons, armor, or items. If you don't want to unbalance future vampire encounters, they can have specific bonuses only against this specific adventure's boss. Example: High-Damage, gas-powered, blessed, stake throwers that are +5 vs. Fester the Vampire, typical crossbow stats otherwise. These can be delivered to the PCs from helpful villagers, unexpected rewards for various encounters on the way, or even a mini-quest the players follow once they discover that a vampire is the upcoming opponent.
- Helpful environment. The final battle might be in a church (plenty of crosses that damage the enemy and cisterns full of holy water), surrounded by daylight, with holes recently scattered through the roof making beams of damaging rays everywhere that the players can push the vampire into.
- Preparation time and knowledge. It never ceases to amaze me what players themselves can do to give themselves the advantage if they know exactly what situation is coming up. They dig holes for gaining surprise. They lay traps. They acquire useful gear. They research obscure lore. I like to reward their creativity since the players will feel like they earned it when that creativity leads to victory.
- Strength in numbers. A hoard of fairly weak allies can easily overcome a stronger opponent. Let the players hire mercenaries for the final battle in exchange for a share of the treasure. Have the oppressed villagers offer to join the final battle (but have them arrive fashionably late). Perhaps the PCs rescued some of the vampire's thralls along the way, and they offer to join the fight in the finale (once they've recovered from their ordeal).
- Deus ex Machina. This is my least favorite, since it doesn't really allow the players to feel involved. Literally, "God in the Machine", this mechanism can be anything the DM wants, and the players didn't expect. Perhaps after the cleric prays, he gets a boon from his Deity of a holy mist that fills the area, damaging the vampire each round and healing the PCs. A powerful, unexpected ally can show up, drawing the vampire's attention at the last possible minute, allowing the PCs to get free hits. The vampire could accidentally trigger an unseen trap that renders him immobile for a few rounds. Note: this is one way to weaken the vampire directly. You can simply arbitrarily nerf the damage it does, or its attack rolls. The possibilities are endless. Suggestion: use sparingly and try not to let the players know what is going on so that you preserve the mystery. If the Deus ex Machina is obvious, players will recognize that they didn't earn these kind of advantages, and will grow disillusioned after a while.
In summary, you can change the Encounter as well as the Monster itself to modify the battle's level rating. You can use your imagination, but players will feel happiest when they contribute to the solution in some way, rather than just having the DM solve the dilemma for them.
