Wisely, “Mad Men” star Jon Hamm and his longtime girlfriend, Jennifer Westfeldt, waited until President Obama had left Martha’s Vineyard to enjoy a little R&R on the island. The couple (right) and a bunch of their friends showed up at the Chilmark General Store shortly after noon the other day, and chaos ensued. (OK, maybe chaos is too strong; the lunch counter got crowded.) We’re told Westfeldt wore a floppy hat and Hamm, like just about everyone else on the island in July and August, was unshaven and disheveled. Later, the couple was with a large group on a private beach in Aquinnah. The pair eventually peeled off and took a long walk down the beach hand in hand. We’re guessing Hamm has spent at least a little time with Vineyard resident Larry David, his costar in the new HBO movie, “Clear History.” The movie is set on the island.