From FortiOS 5.6 the default inspection mode is Flow-based. Useful to apply application policy, but if you do VoIP inspection (particularly SIP…) you have more option in Proxy mode.
The change has no impact on vdom infrastructure, you have to pay attention to some policies that are changed based on inspection mode.
So go under Global > System > VDOM, select Edit and click on Inspection Mode Proxy. Finally, Apply.
Very important, access
fortigate-lab01 # fortigate-lab01 # config vdom fortigate-lab01 (vdom) # edit testVDOM current vf=testVDOM:9 fortigate-lab01 (testVDOM) # diagnose sys session clear fortigate-lab01 #