Re: Xbox 360 - Use wired and wireless options?
Im not certain how the xbox handles the wifi adaptor, either as a replacement for or in conjunction with the ethernet port.
But, in order to get it working the way you want, you will probably have to put the media center and the xbox on different network subnets otherwise it'll get confused about which interface to be sending the packets over. In effect the xbox will be connected to two seperate networks.
Personally i would just use the wired connection to connect to both the vista media centre and the router through a switch if necessary, or heaven forbid - network bridging ( as this would mean the vista machine has to be on in order to connect the xbox to the internet )