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A Carnifex assaults a whirlwind and somehow only immobilizes it. A marine squad then counterassaults the Carnifex. In the marine player's turn, can the Carnifex allocate its attacks to the whirlwind, because it is considered to be in an assault now? - Oaka

Nope. You can only attack a vehicle without a WS by assaulting it via the rules presented in the rulebook. You have to declare a charge against the vehicle, move into base contact with it, and then you get to make your attacks.

After that a unit is not considered locked in combat with the vehicle. So while the Carnifex may be locked in combat, he isn't with the Whirlwind.