Maybe just a bad day? Not hitting the ball well. That can happen to anyone, including Serena, Federer, Nadal Djokovic and I am sure it happened to Graf, Navratilova & co as well.
Plus, Vinci isn't an easy opponent. She had an unusual game. A lot of slices, variations, was often going to the net. When she was in good f and playing well, she was tricky. And if she was playing her first Slam Semifinal, it means she was most certainly playing well. Typically the kind of player who can push you to make plenty of UE and self-destruct if you're in a bad day.
Plus, if I remember correctly, it was a hot and very windy day. Everything Serena hated. Playing against someone who abuses slices in windy conditions is never easy, even for Serena.