To install you need OpenIV, then just open vehicles.img and make sure you are on edit mode and then drag and drop the files in there.
Then open handling.dat and find Vigero2 and change the line to:

VIGERO2 1365.0 1.00 88 0.0 0.08 -0.5 0.0 4 0.216 1.0 177.0 0.18 0.700 0.40 32.0 1.17 0.97 15.5 0.15 0.4832 1.6 0.7 0.8 0.14 -0.16 0.003 0.50 1.0 0.6 1.2 1.5 0.0 18000 440000 0 0

Enjoy.

Credits

Amistech Games- for making the car
GTA Mods Del Norte- for porting the car over to SA
CarCrasher730- for porting the car from SA to IV