Okay, first of all I do not have a VPC in my car, but just like you I noticed that everyone seems to have one so I went through the trouble of figuring out just what exactly it does.
Here's what I've gathered so far... I'm no expert so please take this for what it's worth.
The VPC converts the car's airflow metering from mass air to speed density. This conversion allows for the removal of the mass airflow-metering device, which is a bottleneck to airflow. As far as I can tell this becomes increasingly critical at or above 500 horsepower.