The OpenGL wiki has me confused here:
Indexed rendering, as defined above, requires an array of indices; all vertex attributes will use the same index from this index array. The index array is provided by a Buffer Object bound to the GL_ELEMENT_ARRAY_BUFFERbinding point. When a buffer is bound to GL_ELEMENT_ARRAY_BUFFER, all rendering commands of the formgl*Draw*Elements* will use indexes from that buffer. Indices can be unsigned bytes, unsigned shorts, or unsigned ints.
The index buffer binding is stored within the VAO. If no VAO is bound, then you cannot bind a buffer object to GL_ELEMENT_ARRAY_BUFFER.
You CAN'T bind to GL_ELEMENT_ARRAY_BUFFER unless you have already bound a VAO?! This makes no sense to me as one would naturally want to bind to GL_ELEMENT_ARRAY_BUFFER when filling it in and not want to be worried about binding an associated VAO. Is there another way to fill an index buffer? Does having 0 bound as VAO count as having a VAO bound?