You can be a VIP at Madonna’s concerts, but just don’t try tracking her down when you get there. Fans who paid $2,500 for tickets to the Material Girl’s MSG gigs this week — as part of an “Ultimate VIP Package” — had to sign waivers agreeing they wouldn’t “wander off in any way” during an exclusive concert pre-party at the Garden or “seek out Madonna or her crew.” Fans also had to “agree not to engage in any illegal or inappropriate activities” or “acts that are considered disturbing.” Then again, some of the star’s own acts onstage were considered disturbing by audience members. “I was sitting in the first row and when Madonna did a split in a thong it was a Sharon Stone ‘Basic Instinct’ moment!” said one VIP. Another added, “Yep, I saw the fern.” Most of Madonna’s exposure was intentional, however: “[I’m] showing [my] naked ass for Hurricane Sandy victims,” she quipped as she mooned the Garden crowd both nights, encouraging fans to donate money by throwing it onstage. “One guy threw a $20 bill, and Madonna threw it back and said she wanted hundreds,” a fan told us of the singer’s raunch-for-a-good-cause campaign. “She yelled, ‘Is that all my ass is worth?’ ” (Sources tell us Madonna picked up $60,000 for Sandy victims at her MSG shows between the onstage plea and contributions from her production crew and others.) Spies also said fashion fotog Steven Klein was shooting Madonna on stage during Tuesday’s show for a future project. Spotted grooving at the gigs were Anderson Cooper, Rosie O’Donnell, Kelly Ripa and Mark Consuelos, producer Mark Hudson and singer Madame Mayhem. Meantime, Madonna has been doing her part to help Sandy victims off the stage, too. She, Michael Stipe and Darren Aronofsky were among volunteers over the weekend who helped MoMA’s Klaus Biesenbach distribute respirators, gloves and boots in the Rockaways.