vafc intercepts your vtec signal and map signal to fool your ecu.

since the vafc intercepts the signal your ecu won't see or engage vtec until the set point that the vafc sends the signal out to the ecu. like was stated above, if you set it too low it won't engage until enough speed and oil pressure has built up.

in terms of the air/fuel it intercepts the map signal and the ecu thinks it is at a higher or lower vacuum. your ecu fuel and timing maps are determined by the rpm and vac. by intercepting the map signal and fooling the ecu, it will shift you in these maps effectively giving you more or less fuel depending on which way you go + or -. now the problem with this is it also shifts you in the timing tables as well since they also are determined by rpm and vac. by doing this you are effectively shifting yourself in the timing tables as well.

it's ok for small adjustments, but when you start really shifting one way or another it's control is less controllable so to speak.

if you're looking for a real tuning option i would suggest a tuneable ecu system instead of a piggyback.

what kind of car do you have?