In its most far-reaching decision in more than 50 years, Silicon Valley’s largest water provider will vote Tuesday on whether to embrace or reject Gov. Jerry Brown’ s $17 billion plan to build two massive tunnels under the Sacramento-San Joaquin River Delta. The Santa Clara Valley Water District, based in San Jose, is considering contributing at least $620 million to the project — more than $1 billion when financing costs are included. The vote could shape whether the project is ever built or if it is reduced in size.