The party should not understand why the stones are needed. They should fail to get the first stone no matter what and then find that several tomes are missing from the Collegium library. These tomes deal with particular aspects of blood magic, an old and nearly unused sort of magic due to the high cost of casting ritualistic spells that could go wrong easily.

Then the party goes to investigate a report of activity wherever the second stone is. The stone is already gone, but by now they should be realizing that there is something very wrong going on with the stones. A person at the Collegium can remember a rumour of the stones, the Bloodstones, and that there are four of them. They were scattered to avoid the concentration of power that could be gathered were they all in the same place. Yes, he confirms they are for blood magic.

Then the party gets to try to race to the locations of the other two stones, in hopes of getting them and bringing them back to the Collegium so they can't be used for nefarious purposes. They should fail the first time, and then the second time they should triumph and take the stone back, fighting their way the whole way back (the stones can't be teleported or otherwise magically moved) to the Collegium.

When they drop off the stone, they are rewarded and commended. Then, and only then, do they realize something is terribly wrong. The other stones were being gathered deep under the Collegium all along, and the very person who told them about the stones is in league with the vampire. The entire pitched battle that they fought over the last stone was actually skewed so that the party would win and think the danger had passed.

The way I DM, I'd personally have it found out too late - the ritual is made and the elven forest is destroyed by the ravages of a mass of undead, and the reason is that Collegium member made a deal with the vampire - you help me get the stones and I'll use them to destroy the forest's defenses. The real danger, however, is what he does next to try to become a god, or a lich, or some such thing.

It would involve desperately trying to save some of the elves from annihilation (the forest and their cities would be destroyed, though) and then going back to figure out what happened, with a lot of treachery and a battle in the Collegium, only to find the stones had been moved, tracking down the mad Collegium member to his real lair and a massive epic battle at the end.