When I was V.P. of my sorority, I was in a similar predicament. I had no job description and had to create one for myself. In addition to assisting the President, I organized and oversaw the minor board which included publicity, social, sisterhood, fundraising, and any other position not on the Executive Board. We had weekly minor board meetings where I made sure these officers did their jobs and delegated like crazy. The president of my sorority told me that I did more work than she did!
Good Luck!!