Idle in avast! means not moving the mouse. Even if CPU is 100% used and you’re not oving mouse,VRDB will start to generate. In this case its better to use screensaver mode (and don’t forget to set some screensaver).
VRDB is also fully integrated into avast!,so you cannot remove it or disable it so it would use less resources. It can only be disabled in way it will no longer generate its database.

Network Shield is an addition to classic software or hardware firewall and not a replacement. It catches network worms and exploits like Sasser or MSBlast.