Can Disallow prevent me from crewing a vehicle?
Yes, Disallow will prevent you from crewing your vehicle: your vehicle will not become a creature, and the creatures you tapped to pay the crew cost will remain tapped.

From the comprehensive rules on Crew:


  702.121a Crew is an activated ability of Vehicle cards. “Crew N” means “Tap any number of untapped creatures you control with total power N or greater: This permanent becomes an artifact creature until end of turn.”


This is an activated ability with a cost of tapping creatures and an effect of turning the vehicle into a creature. It's a legal target of Disallow, so we can prevent its effect from resolving.

Note that even despite the crew ability being countered, Veteran Motorist's ability would still trigger (because you tapped it to pay a crew cost), and the vehicle will still receive +1/+1 until end of turn. If you manage to turn the vehicle into a creature later this turn, that +1/+1 will start to apply.