If vsync is enabled, and your monitor is displaying at 60hz (which people should really change, as it's bad for your eyes unless you have an lcd), then the game keeps the rendering in sync with the monitor refresh: 60fps (or lower), which limits the CPU usage to whatever it takes to render 60fps, and limits the packet rate to 60pps, because as it stands, the packet rate can't be higher than the framerate.
Take vsync off, and it just taxes the system resources to the max to render as many frames as possible, even if you can't see them, thus more CPU usage. If your cl_cmdrate is set higher than 60, then you'll be sending more command packets, and thus use more bandwidth.