NEW YORK (Reuters) - Around 200 U.S. Army veterans and Vietnamese people demonstrated outside Democratic presidential front-runner John Kerry (news - web sites)'s New York headquarters on Saturday to protest his opposition decades ago to the Vietnam War. Waving American and South Vietnamese flags and singing the U.S. national anthem, they held up signs saying "Hanoi John," and "Kerry Betrayed Vietnam Vets." "We won't sit by and let the American people think that we are going to stand by somebody who stabbed us in the back," said Jerry Kiley, a veteran and one of the protest organizers. Kerry, a Massachusetts...