Vittana, which beat out such well-known charities as DonorsChoose and people like Mayor Michael R. Bloomberg of New York, is building a student loan business for microfinance institutions that cater to poor people in the developing world.It uses a model similar to that of Kiva.org, a nonprofit organization that funnels loans to individual borrowers through microfinance institutions. The method is to solicit individual lenders for money that will back loans to young adults seeking college educations.