However, you could use a descriptive phrase like:
* Fleet of vans
* Group of vans
* Line of vans
* Convoy of vans
* String of vans
You can also get creative and use a metaphorical collective noun based on the context:
* A swarm of vans (if they are moving quickly and in a chaotic manner)
* A pack of vans (if they are used for delivery)
* A crew of vans (if they are all working together)
Ultimately, the best collective noun will depend on the specific situation and your desired effect.