Before. Even though an Immobilization TC is said to be "immediate" (D5.5), this means it is resolved immediately after the effects DR vs the vehicle. The Collateral Attack, OTOH, is
simultaneous to the original attack and uses the same DR. There is an existing Q&A:
Q. (C1.55, D.8, & D5.5) A CE StuG is attacked by OBA resulting in a K/3 (hull hit - immobilization) versus the AFV and a 2MC versus the CE crew. Does the Immobilization TC take place before the collateral attack versus the CE crew?
A. The collateral attack occurs first.