Calling Gov. Jerry Brown’s ballot proposal to temporarily raise taxes a “risky venture,” Vacaville Unified School District’s chief business leader on Thursday told trustees that the 2012-13 district budget will remain in flux, as several recent annual budgets have, when the next school year gets under way.Kari Sousa, interim assistant superintendent of business and administrative services, said Brown’s $92.5 billion state budget, announced earlier this month, assumes that voters will OK Brown’s initiative to raise income taxes on people who make $250,000 a year or more and boost the state’s sales tax by a half cent.