I wouldn't recommend the vans, but I gues you're not going to want to split the family up between more than two cars (depending on age). Distance is also a factor, we had c. 90 miles to LHR so I didn't want the Vito!
I ordered two cars (Mercedes E300 Hybrid, saloons) and we only just got our 4 large suitcases and two small carry on's in them.
There was no problem booking two, but they were booked seperately.