VIEQUES, Puerto Rico — Another long holiday weekend is starting, and tourists are flocking by ferry and small plane to Vieques, a tiny island off the coast of Puerto Rico that’s famed for bright turquoise waters, lush mangrove forests and picturesque free-roaming horses. In an empty lot near the $500-a-night W Retreat & Spa, a man with a gun is stalking some of those mares.