The Alternate Absurd Theory
It would take too much space to requote many of these arguments, so I'll just state that I really don't like the "Granny knew all along that her plan would work and manipulated everyone from Oats to Agnes into believing she might feel" theory.
Not because it isn't plausible (it is) or because there isn't evidence to support it (there is), but because it's not particularly dramatically compelling. If we believe in this theory, then essentially Granny Weatherwax is a superhuman or almost a demi-god creature who manipulates her friends as well as her enemies to achieve the outcome she wishes to achieve. (Well, she does do this, but not to such a totally risky purpose.) If we believe this argument, then there is nothing to admire about her unless we admire Machiaviellian manipulation.
However, if we go to the beginning of the book, we already see that Granny is feeling emotionally vulnerable. She thinks she hasn't been invited to the baby naming. She feels that she's not appreciated. She feels that the Lancreans resent her for making decisions of life and death over others. She is not even quite sure whether she is a totally "good" person anymore. She is full of doubt. And perhaps part of this doubt is what made her vulnerable to the first attack of the vampires at her cottage. The "100% confident" Granny would have known that her invitation got stolen by a magpie.
But, in all the previous books, Granny is never seen as a manipulator. She is always a straight shooter who always speaks her mind and doesn't put her friends' lives at risk just for the sole purpose of riding in at the last minute to save the day.
I choose to believe that she ran away from Lancre because she really didn't think she could beat the vampires and that, because she had felt rejected by Lancre, that she wasn't even willing to try. It was only when the witches came after her and showed her the invitation that she realized how needed she really was. She may have told the witches to to off and fight the vampires by themselves, but she did this so she wouldn't have to be distracted from her own plan (which Nanny did inadvertently clue her in on) to protect the others.
She didn't know whether her Weatherwaxing plan would work, and when she first appears at Lancre, dirty and disheveled, it's clear that she's been fighting the vampire's influence (and perhaps her own self-doubts) all the way. She tests the Count at first, trying to use her usual headology, finds that this doesn't work, and then finally sacrifices herself for the good of the community, not knowing what the result will he. Is there Christ allegory here? You betcha! (Disclaimer: I'm neither Christian nor or a believer)
Metaphorically, we can interpret her whole journey with Oats as her struggling for the preservation of her soul. Perhaps she'll never become a vampire, but the presence of Death along the way demonstrates that she may lose her life. It's Oats' faith, in her, along with her own faith in herself and commitment to saving her community, along with the support of the Pheonix, that keep her going.
That she wins in the end proves that she has not only "redeemed" herself but she has also instilled morality (even if it's forced) into the Magpyrs. Again, if we want to get all biblical about it, we can find comparisons to Jesus's statement of "This is my blood you drink." It may not last long, but is there for long enough. Granny has not only won the physical struggle against the vampires, but her own personal moral struggle as well. The Phoenix confirms that she is a "good person" and this gives her the spiritual strength to triumph over the vampires, although, in the end, we need Oats to finish off the Count.
For me, this is a far more satisfying and emotional reading of the story, and it leads perfectly into the Tiffany Aching series. Dare I say it almost hits C.S. Lewis territory? (Disclaimer: not a big fan of CS Lewis.) Whatever Pterry's views on the existence of roundworld deities, it's fair to say that this is possible his most "spiritual-themed" DW book (probably even more than Small Gods, because in the latter, Brutha knew the god existed. In CJ, neither Oats nor Granny are supported by the PHYSICAL presence of a deity).
I should add here that one of the strengths of this book is that PTerry leaves enough room for either interpretation. In fact, he makes an explicit reference to the potential for this kind of debate when the witches leaves Granny in the cave. One of the witches (forgot whether it's Magrat or Agnes) asks if Granny had faked her whole 'confession of weakness' just to get the other witches to all work together. Nanny replies, "That's soggy thinking."
Interpretation off.
J-I-B