A historic vote on the Delta tunnels project is getting a do-over. Southern California’s powerful water agency — the Metropolitan Water District — said Thursday its board will vote again in July on whether to pay for the lion’s share of the project, known officially as California WaterFix. The announcement comes after environmentalists and an open government group complained that Metropolitan directors violated the Brown Act before voting in April to support the tunnels. The Brown Act sets rules intended to prevent government boards from making decisions behind closed doors.