Bill Charlap, the brilliant pianist and artistic director of the 92nd Street Y’s Jazz in July concert series, is a modest man, almost to a fault. He leans much closer to understatement than to exaggeration. So when he makes a bold, sweeping claim, such as describing the Vanguard Jazz Orchestra, whom he presented in concert on Thursday, as “the foremost large jazz ensemble in the world,” we should be inclined to take him very seriously.