Instead of messing with the vacuum line you can also just unplug the solenoid cable in the cargo area like the 335i guys do. This is what we did to our cars.
Just pull on the handle that removes the rear carpeted panel area behind the driver's side tail light (where the amp is located), you will see the solenoid and ECU cable directly behind and below the tail light, unplug it and enjoy.
Plug it back in before service.
335i DIY same as X3:
http://www.e90post.com/forums/showthread.php?t=80661