My sprinter has been broken into a few times – typical MOs are:
If the rear windows are glazed and theres no bulkhead then pull the rubber seal out of the right hand window take the glass out and reach in.
If have to get into the cab because theres a bulkhead and you are feeling less polite jam a screwdriver through between the bodywork at the top edge of the door handle above the key hole and you can operate the door release through there. This is only polite in the sense that the van will look intact and keep the rain out once you’ve moved it. It’ll still be an operating door that you can lock, just one that anyone else who wants to can now open with a screwdriver or you made the hole big enough- their finger
Infact – look and see if this has already been done in the past, sprinter locks are commonly attacked this way
Less polite still is a brick through the window – which will make it more urgent for him to collect his van but will be cheaper and quicker for him to repair
To move without getting inside maybe chock the wheels, crawl under and slacken off or cut the handbrake cable perhaps.
For all of this – the steering lock will still be on, so you’ll only be able to move it in straight lines ( or in whatever arc the wheels are turned at)
EDIT Actually you could probably pull the seal out of the quarterlights to get into the cab