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.
I also heard it gains around 20RWHP....but what I want to know is the difference between the stock MAF measurements compare to VPC's speed density measurement. would that be more accurate? does it help balancing out A/F ratio or fuel curve in anyway?
is it more of a single turbo "must" mode instead of BPU?
Both mass air and speed density measure the same thing in different ways. I don’t believe that either method is more accurate than the other. However, one method allows for more air to flow through the intake than the other. Namely, by switching to speed density you are in effect removing an inhabitance to airflow - i.e. the mass air meter.
The mass air meter is a physical protrusion residing along the path of your engine’s intake. By removing this protrusion you allow more air to flow through your engine’s intake.
This is one of those things that would take about 30 seconds to explain if I could point out exactly what I’m talking about, but could take forever to explain in words. I hope this helps.