WASHINGTON — Late last year, Agriculture Secretary Tom Vilsack strode into the Oval Office to tell President Barack Obama that he wanted to resign.“Mr. President,” he said, “I think it’s time to go.”Vilsack had survived nearly eight years in Washington, D.C., as Obama’s model Cabinet secretary — a disciplined and efficient technocrat who understood the inner workings of his department, worked well with lawmakers and did not cause trouble for the White House.Lately, though, that approach did not seem to be enough to fix the problems he was seeing in the country.