Igor,
Each vlan has to have its own ip address. If you want to have a loopback address for the whole switch then:
- Select one of the vlans in the switch and enable loopback-mode
- assign loopback ip address
- assign ip addresses to all other vlans
- enable ipforwarding in all vlans
If you want to have a loopback IP address that does not belong to any of the existing vlans, just create a vlan with no ports, enable loopback-mode and assign an ip address.