To clarify, you can create your own tilesets such as Mack's revamped ones that many use now days; the only thing is that you're limited to five per project.
To do this, you simple rename your tileset, which needs to be formatted in VX RTP, to TileA, TileB, TileC, TileD, and so on. If you are satisfied with what VX supplies you with, you will always have TileE which is left empty for custom edits.