Configuring the speed is usually done by autonegotiation. If there is one nic on the subnet which only supports 10Mbit then all the nics on that subnet will have to switch to 10Mbit.
Check the following page for the options you can pass to the module when loading it:
http://www.scyld.com/network/vortex.html
I don't know switches but I think you have to set the port on the switch to 100Mbit too if you don't want it to auto negotiate.
Mik