Ok, BMW OEM wheels are 20x10 ET 40 front and 20x11 ET 37 rear, Vossens are 20x9 ET 20 and 20x10.5 ET 27.


You can make this work using 20mm spacers for the front and 10mm spacers for the rear, also you have to change hub diameter from X5's 74.1mm to Vossen 73.1mm, so for about $190 you can order such spacers from one of those places:

http://www.motorsport-tech.com/4DCGI/wheel_spacers