I should have mentioned to make sure the belt is intact and on the pump pulley as this would also cause the problem but i doubt it's that, It's the power steering pump, it's driven off a belt by a pulley, there are pipes going into it which lead to your steering rack and resevoir. It's diy'able just a bit messy. Disconnect the pipes first (just at the pump) then slacken the belt tensioner and remove the belt, then remove the bolts securing the pump. Refitting is a reverse of removal then top up the resevoir and start engine, move wheel from lock to lock then check fluid level. If you search ebay for a vivaro pump you should find a picture of one, If reliability is a must buy a new one, but if you don't mind getting your hands dirty a used one may last however long you're keeping the van.