Just tell them its more of a security measure in case the vans go missing , it will be easier to get them back , and you could add that its a requirment on fleet insurance .
I had a van go walkies for a week once , i called the old bill but they could not class it as stolen as i had given the keys , needless to say when i found the van and him he no longer had a job , he was lucky he still had teeth .