The van is already out
Two jobs are scheduled on Wednesday morning, both assuming Van 2. The second crew finds out in the yard.
Make each vehicle and each shared piece of equipment a sub-calendar that refuses overlapping events. A job that needs a van already booked is refused when it is saved, with the job it collides with, so the office picks another van while the crew is still at home.
Van 2 is out on a boiler service, so the gutter job is refused and goes out with the trailer.