Failover License :
Designed for use in conjunction with a similar Cisco PIX Security Appliance model that has an Unrestricted license, providing a cost-effective, Active/Standby high-availability solution.
Provides the same capabilities as the Unrestricted license, except the Failover license does not support Active/Active failover. Requires presence of similar Cisco PIX Security Appliance model with an Unrestricted license to operate properly.
Yes, you can have VAC+ with FO unit.
Refer this link:
http://www.cisco.com/en/US/products/hw/vpndevc/ps2030/products_data_sheet09186a00800b0d85.html