Vermiculite isn't inherently harmful; you can still buy it at garden stores. But 70 percent of the vermiculite sold from 1919 through 1990 came from a mine in Libby, Montana, which shared the space with a deposit of asbestos. If your home was built before 1990 and has vermiculite insulation, you should assume it's contaminated with asbestos, which according to the EPA puts you at risk for lung diseases, including asbestosis, lung cancer, and mesothelioma.
If you have it: Don't mess with it. There's less risk in just living with the material than in trying to remove it and possibly releasing asbestos into the air you breathe. Rethink storing boxes in your attic if accessing them disturbs the vermiculite. And if must-do renovations require cutting a hole in the ceiling, call a contractor certified to deal with asbestos.