In the story "Daisy Miller" is she a vulgarian who corrupts high standards of culture OR is she simply a naive girl corrupted by European decadence? What textual evidence makes readers feel either way? In other words, is the evidence in the text or a product of readers' own attitudes/prejudices?

Henry James is often considered a master of nuance in fiction. He rarely shows readers just one side of a question, but rather displays fully rounded characters in all their complexity. Thus, the answer to the question is that James is not presenting us with an either/or dichotomy but rather a character that evokes complex reactions that reveal the viewpoints of other characters and readers.
Daisy Miller is an independent, innocent, spontaneous, and unconventional young woman. She is uncultured and ignorant and at times seems rather self-centered, oblivious to the opinions and desires of others. In many ways she represents the United States as a country. While some characters find her refreshing and admire her beauty and innocence, others deplore her lack of culture and insensitivity to local beliefs and customs. Even though from a feminist point of view, her insistence on walking after dark alone and associating with Giovanni should be her personal choices, they still display insensitivity to her social circle.
Also, Daisy seems crassly materialistic and rude, talking about Paris mainly in terms of clothing and seeing social circles entirely in terms if herself. At the castle, Winterbourne "saw that she cared very little for feudal antiquities and that the dusky traditions of Chillon made but a slight impression upon her."
Thus James seems to be letting readers see that the expatriates who are striving to be obsessively European, assimilating to Europe's culture and manners and prejudices, and the brashly ignorant Daisy are both unbalanced in their approach to social interactions and is maybe suggesting that a happy medium would be preferable.