My first suggestion, which I hope works, is nice and simple. The fault maybe related to dirty contacts that connect the door lock electrics to the body of the van. Try pushing the door inwards as hard as you can whilst pressing the unlock button on the remote. If you are lucky this may just help make a better contact and supply power to the door lock mechanism.
If this does not work, work from the inside and take off the inner panel of the side door, locate the mechanism and try to manipulate the lock levers and rods with a suitable tool. The electrics to the lock could be ruled out if you can actually see or hear this working when remote control is being pressed. If you are sure the lock is working, pull the handle to observe which internal lever is being activated, then by giving this lever a shove, beyond the amount it is moving at the moment, could unlatch to door.