Workers for a nonprofit that runs soup kitchens, shelters, drop-in centers and a food pantry said Friday that employees in Bangor, Portland and Lewiston have voted overwhelmingly to form a union. The workers voted 90-11 to form a union represented by the Maine State Employees Association-Service Workers International Union Local 1989. The employees said they sought to unionize to improve services to their clients and increase their wages. In recent years, Preble Street has experienced high employee turnover as workers burned out or found better paying jobs elsewhere.