The Vec allows you to adjust for Forced Induction, different sized injectors, individual injector pulse widths, etc.
The Vec atually allows you to adjust the tune for the modifications to the engine's demands for timing and fuel which allows the engine to make optimum hp. The Vec allows adjustments not supported by the factory ECU. Forced Induction needs completely different programming for the boost pressures in the intake and the Vec has an internal MAP sensor that allows tuning according to Manifold pressures and loads.
Larry M (Macedo?) you can listen to what he says since he has vast experience in this field.
Ted