I agree with the previous answer, but would to point out that literary criticism and decoding texts are never straightforward and simple. True, Faulkner was a southern writer and the motif of the decaying south is certainly a valid way of reading the story. However, Faulkner's texts in general always exhibt a preoccupation with kinship ties as well. These ties, or family bonds if you will, form the microcosm of the south. This holds true for many of his other works such as his stories "The Bear", "Wash", his novels The Sound and The Fury, Absalom,Absalom!, and Light in August. This occupation with kinship leads him time and time again to examine different notions of the female, sexualized body in contrast to overarching family and societal structures. The female with her ability to become pregnant and carry on the patriarchical family line is always vulnerable in his texts. She is vulnerable to early sexual rejection and later doomed to spinsterhood ( Ms. Emily). If she is black ( and Faulkner uses his characters over and over again in his stories and novels and has created a whole family line of mixed race characters) she is vulnerable to miscegenation, or race mixing, even though the story may be narrated through the conscience of a white male. Another trait that Faulkner's females have in common is rejection. They are quite often jilted, deserted, sexually abused, lonely, and often die. It is thus never the wholesome American family he strives to portray, but with typical modernist fervor he sets out to dismantle the institution of the family and show how it is decaying before our eyes.

That being said, I  think it is fair to claim that Emily's vulnerabilty stems from simply being a woman. A woman in one of Faulkner's writings...

Miss Emily best represents the theme of vulnerability but in doing so she does simply symbolize an old maid longing for a knight in shining armor.  Faulkner uses Emily to illustrate the vulnerability of the Old South and all its traditions.

First, Miss Emily is vulnerable when her father still lives.  Their social status in the town coupled with her father's intimidation of her suitors (he stands in the doorway "clutching a horsewhip") leave Emily with little hope of finding true love.  When her father dies, the townspeople again pity her and wonder what will become of her especially when she refuses to accept that her father is dead.  In the end, Miss Emily proves not to be so vulnerable because she "conquers" her suitor by poisoning and hiding her secret for years.

The Old South is the true vulnerable character in the story.  Miss Emily symbolizes the traditions that the Southern aristocracy held so dear--special privileges because of one's name, family houses passed from generation to generation, owning "servants," etc.  As younger generations take over Jefferson, they refuse to keep up these traditions.  For example, they approach Miss Emily for payment of her taxes when traditionally she did not have to pay taxes.  Likewise, Homer Barron, Miss Emily's fiance, invades the small southern town.  As a Yankee he comes down South for work, and the townspeople and Emily's relatives are horrified that she would be seen with a Yankee commoner.  It is simply another intrusion on their Old Southern traditions which increases the vulnerability of their way of life.