Just had a good drunken idea about how to reconcile Toughness with how vehicles take damage in 40k.
Rather than a Toughness characteristic, vehicles would have a Durability score. Wvery time they took damage you would roll versus their durability score. If passed then they were durable enough to shake it off much like biologics and Toughness. If they fail they reduce their Durability score and keep moving. This represents a vehicles ability to shrug off damage but also slowly accumulate damage as well.
Borrowing a mechanic form Deadlands RPG, We could add in steps to the Durability characteristic where for each mutiple of the step, the vehicle would add in a mishap (basically a roll on the vehicle damage table). For example a Durability rating of 4/9 would mean that a vehicle could fail 9 Durability rolls but at the 4th and 8th failure they would get things like weapons blown off or immobilized.
I know that drunk me has not thought this through very well but it is still an interesting thought.