As far as i can remember, there are only two endings, good and bad. I've never even heard of a grey ending, and i finished that game at least a dozen times.
To be able to finish the game as a goody two shoes, your humanity needs at least to be above 50, which is quite easy to establish. Simply don't suck any innocent humans completely dry, and.....
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Appose to prince Rudolph Brandl in Prague. He'll hit you with a spell that raises your humanity up to 80. That gives you the opportunity to drain the blood from Lucretia's heart and gain her power, and still remain with plenty of humanity to end the game the good way.