In great detail, why would the authors of the Legend of White Snake depict Bai as vulnerable despite her powers and wisdom?
Bai is likely depicted as vulnerable despite her powers and wisdom because such a quality quite literally humanizes her. To better understand this concept, let's consider the plot of the story:
The young man Xu Xian buys tanguyan from a peddler and consumes it, not realizing that what he has really purchased are magical pills that will make him immortal. He throws up the pills into a lake, and a white snake spirit consumes them so that she may gain the immortality and spiritual powers they offer. This act bonds her to Xu Xian. Shortly thereafter, the white snake transforms herself into a female human in order to rescue another snake from a beggar. In her female form, the white snake takes the name of Bai Suzhen. The green snake follows suit, becoming human and taking the name of Xiao Qing.
Xu Xian and Bai eventually meet, fall in love, get married, and open up a medicine shop together. However, Bai's secret identity is revealed to Xu Xian during the Duanwu Festival, when Bai consumes realgar wine that shows she is not really a human but a snake. Xu Xian is so disturbed by this revelation that he has a heart attack and dies.
In her loyalty to him, Bai takes on a dangerous quest to the Four Sacred Buddhist Mountains of China in order to seek out a way to bring Xu Xian back to life. She does so, bringing her husband back to life, and she is delighted to find that Xu Xian still loves her despite her non-human origin.
We see this vulnerability show up again when Xu Xian is imprisoned and Bai must once again strive to save him. The fact that she is now pregnant with his child, however, has weakened her spiritual powers. She is unable to save her husband, and Xu Xian must find a way to escape on his own.
In other words, the authors of this tale are likely shaping Bai as a vulnerable character in order to do several things:
- make a point about the fragility and fallibility of human life, which so swiftly can change and even end
- make a point about the softness and weakness of traditional representations of the feminine
- demonstrate that strength and immortality are divine qualities and that in becoming human, Bai assumes great risk and distances herself from her spiritual abilities
- represent Bai as both the "yin" (feminine) that disrupts the normal order of human life in her shift from snake to human AND as a hero who seeks out balance through reunion with the "yang" (her husband and son)
- make Bai and her quests throughout the story relatable to a human audience--particularly a traditionally Chinese one that may search for understanding of her journey through identification with Daoist traditions and allegory.
Overall, it's also key to recognize that this story has evolved in its tone over time; while it began as a cautionary tale in which the white snake lady causes the downfall of a moral man, it has shifted into a more romantic one that demonstrates Bai's actions as motivated by deep love. Thus, we might finally add that for a modern audience, her vulnerability is a profoundly beautiful facet of the tale.