When President Obama returns to Martha’s Vineyard next month, he won’t be taking up residence at Blue Heron Farm, the Chilmark estate that the POTUS and FLOTUS rented during their three previous Vineyard vacations. That’s because the 30-acre spread is owned now by British architect Lord Norman Foster and his wife, Lady Elena Foster, and the couple isn’t interested in renting it out, even to the leader of the free world. But the couple, who paid $21.9 million for Blue Heron Farm in 2011, did invite a few hundred folks to the house over the weekend for a $250-per-person fund-raiser benefiting the Martha’s Vineyard Playhouse. The event, which raised over $100,000 for the theater’s renovation, was the Fosters’ first public event since paying the hefty sum for the property, and the crowd included ART founding director Robert Brustein, Harvard professor Henry Louis “Skip’’ Gates Jr., Tony Award-winning librettist James Lapine, poet Rose Styron, director Diane English, Harvard professor Alan Dershowitz, art collector Olga Hirshhorn, actress Brooke Adams, Melinda Farrelly, wife of filmmaker Peter Farrelly, home-improvements honcho Bob Vila, and “Says Who’’ hosts Arnie Reisman and Paula Lyons. Guests were impressed with the place, lingering by the Infinity pool and adjacent glass outbuilding. Foster’s firm, Foster + Partners, may be best known for designing England’s Wembley Stadium, but it also designed the MFA’s Art of the Americas Wing.