Most Vocaloids are given a brief description on their box containers to describe what they are as a product but they are not officially given (or rather not announced) a role or personalities.  This open source allows for producers and the fanbase to create their own personal role for the Vocaloid. There are exceptions given by the companies as they don't want their product to be viewed in a bad light, but other than that, Vocaloids can be given many roles and be open to many personalities depending on a song or fanart work.
For example, Miku has been given just about every role a person can think of. She could be a cheerful schoolgirl in one song or depressed woman in the next. She could be helpful and gentle or she could be mean and heartless.
There are a few exceptions. One was a Chinese Vocaloid named Luo Tianyi, even in her original existence as Yayin Gongyu, she was given a back story and a developed personality. Other Chinese Vocaloids, such as Yuezheng Ling and Zhanyin Lorra, were also given back stories and developed personalities. There is the English Vocaloid named Sonika, who expresses herself by using Twitter, but most fans develop their own version of her.