The new CEO of Houston's only black-owned bank is counting on a budding relationship with Citigroup to expand its product set and boost profits. Laurie Vignaud joined Unity National Bank of Houston in January from Capital One, where she was senior vice president of community development banking. She took the helm two months after the failure of City National Bank of New Jersey in Newark cut the ranks of banks owned or operated by African Americans to 21 — about half of what existed before the financial crisis. Vignaud said she is convinced that the 34-year-old Unity must embrace a new business model to survive.  Unity is one of seven banks owned or operated by minorities enrolled in the Treasury Department's Financial Agent Mentor-Protégé program. The initiative pairs big banks that process financial transactions for the government with minority depository institutions. The smaller banks can earn fee income by taking on some of the workload. A key objective is to keep the smaller banks viable, which in turn will help them continue to address the needs of underserved communities. A handful of large banks, including Citi, JPMorgan Chase and U.S. Bancorp, are working with the minority-focused banks.