'Be not Nobody' did indeed come to Vanessa in a dream. She had originally intended to name her first album 'Rinse', but when this new title came to her it was very symbolic to her, and so she rushed it to the front, insisting on it rather than 'Rinse'. To Vanessa, it stands for that time in her life when she was emerging from darkness and obscurity, and becoming successful at something she loved.
In short, she was becoming "somebody" and no longer being "nobody", hence the imperation, 'Be not Nobody'.