When you create a grid box inside Vanishing Point, in addition to having the option of returning that grid back to Photoshop as a 3D object, you also have the option of exporting it as a 3D object that you can import into After Effects CS4 and manipulate in a 3D layer. Just click on the small right-facing triangle in the top of the dialog, and in the fly out menu, choose Export for After Effects (.vpe).