RE: VDOM logging
Q1; you have support for per VDOM logging, iirc you have to enable and override this from the CLI. I' m sure some one would correct if I' m wrong.
Q2: No this is not correct, if you enable vdom on a production Fortigate, the worst it will make you do is to; logout and log back in.
As with any changes, you want to backup the configuration b4 enabling vdom. All interfaces per default are in root-vdom and enable mult-vdom does NOT change this.