Millions of Bay Area residents could get extra drought insurance against water shortages and quality problems from a proposed $800 million expansion of the Los Vaqueros Reservoir that may have up to 10 water suppliers as partners. Ten water agencies serving San Jose, Fremont, Oakland, Concord, Richmond, Antioch, San Francisco and other communities have negotiated preliminary deals to contribute a combined $1 million for feasibility studies on expanding the reservoir south of Brentwood.