ASA Virtualization context configuration

ASA licensing will allow you or not to use virtual contexts.
In order to use them, will will have first to change the mode of the firewall:

firewall(config)# mode multiple
WARNING: This command will change the behavior of the device
WARNING: This command will initiate a Reboot
Proceed with change mode? [confirm] yes
After the reboot, just create your admin-context
firewall(config)#
firewall(config)# admin-context admin
Creating context 'admin'... Done. (13)

Alocating interfaces:
firewall(config)# context admin
firewall(config-ctx)# allocate-interface GigabitEthernet0/0.101
firewall(config-ctx)# allocate-interface GigabitEthernet0/1.102
firewall(config-ctx)# allocate-interface Management0/0
firewall(config-ctx)# config-url disk0:/admin.cfg

in FWSM you can add Vlans L3 as interfaces:
fwsm(config)# context Internet
fwsm(config-ctx)# allocate-interface Vlan1000
fwsm(config-ctx)# allocate-interface Vlan1001
fwsm(config-ctx)# config-url disk0:/internet.cfg