Binding to server on a separate VLAN

Set up a new network today in an IT lab where the clients had all successfully been bound to a Snow Leopard server for Open Directory authentication and Home Folders. My engineers want to put servers on a separate VLAN with clients on DHCP and the server on static. The clients see the server in the Network Account Server set up (Login Options) - the 'light' went green - but they failed to authenticate. We had to put them all on the same VLAN for now - it works but it is not what we wanted Can anybody help?