You should be able to use the original VLAN and default route (but presumably change the mask). Each VLAN does not need a default route, your device will route just like a router if necessary. However, keep in mind that Auto Last Hop will generally return traffic to the MAC address it was received from and ignore the routing table.
If you're looking to route traffic that isn't handled by LTM (by an SNAT or Virtual Server) you'll need to create some routing Virtual Servers. Let me know if you need further information on that.
Nimbostratus
