1. Did Vermeer love Griet? They had a rather special relationship, and it is obvious that he did give her special attention, but what were his true feelings? Did he just think that she was pretty and that's it? Did he just see her as a distraction from his harried life? Chevalier doesn't let us get inside of his head to answer these questions--we just have to infer things based on the text.
2. Did Griet's character really exist in life, and if so, how much of her relationship in the Vermeer household was fictional, and how much fact? For example, did she indeed end up with the pearl earring at the end?
3. What was Griet's future? Did she do anything with her talent for art and her eye for colors? Chevalier sets her up as a savy artist-in-training, but we never really know if she does anything with that in the future.
4. Was her sweetheart ever aware of her feelings for Vermeer?
5. What were Griet's feelings for Vermeer exactly? We know that she thought of him often, and was uncomfortable yet pleased by his attentions. But did she think of him more?
These are all questions that Chevalier leads the reader to, but doesn't blatantly answer; instead, you are left to fill in the gaps using clues from the text, and your own imagination. I hope that those thoughts helped; good luck!