Well.. I was sorta thinking along the lines of.... well, has anyone played Half-Life 2 and the episodes? Sequences like, for example, in episode 2 where you and Alex are buzzing around in the stripped muscle car.
So someone would drive, the others would pile into passenger seats (1 or 2 vehicles total) and you'd have to make it to your operation point. But..., for example, there's some sort of obstacle which you would have to clear in order to proceed. So you'd have to get out of the vehicle, accomplish some task (obviously, zeds would be in the way) then proceed.
Not so much a defense point, but more or less just a type of progression to sort of freshen things up instead of always being on foot. You could, however, incorporate some feature by which the zeds could destroy your vehicles, making it impossible for you to continue. So at a point that I previously explained the team would have to split up. One to accomplish whatever task needs to be done (say, restoring power to open some door) whilst the other team has to defend the vehicles from being destroyed.