there is no problem running with a single vlan.
You can even assign the unique vlan to 2 physical interfaces. One for uplink and for downlink.
The CSS will bridge traffic between the physical interfaces.
It's easy if you want to insert the CSS inbetween the servers and their gateway.
No need for readdressing or changing the default gateway.
If you only use 1 physical interface and 1 vlan, this is called one-armed mode, and you need to pay attention to the return traffic from server to client. This traffic must go through the CSS as well.
Client is often needed in this situation.
This is accomplished with a group command.
Gilles.