Vermiculite was a great insulator, and relatively fire resistant. It has often been described as being "rock popcorn" because they heat up the mineral to blow it up into the puffy little rocks that they are when you put them in your home. It was a really fantastic and fascinating process at the time that a lot of people don't really consider anymore.
So why is vermiculite not used as much? Because much of it contained asbestos. When it comes down to it, why take the chance when it comes to asbestos. If you have vermiculite in your home, be careful with it and consider having a professional asbestos company handle it for you.