In addition to setting the layer 3 interface on vlan 400, you also need to configure the ip address on this interface you want to use. And to have this as your core, you will need the devices and servers in the vlan use this interface as their default gateway.
Configure the address under: interfaces vlan unit 400 family inet address 1.1.1.1/24
Use the address 1.1.1.1 as the default gateway
You should now see both the loopback and vlan.400 in the routing table and they should be able to be reachable by ping.