The vents are designed to provide an air cooling system to the back of the fridge therefore it is logical that if air is able to enter, then water can also follow. However, under normal circumstances there is little chance of water entering and as it is a heated area from the fridge, the small amount of water usually disappears quickly.
If you have the van facing into driving rain, then yes, I am sure water will get in, just the same as leaving the window open in your house. For damage to have occured must have meant a lot of water over a long period of time, which does suggest some form of negligence. Where there may be cause for complaint is if the fridge has not been installed correctly and the area at the rear of the fridge is open to the inside of the van. This can usually be confirmed by the presence of a draught from around the fridge. However, this does not in itself mean the vents are not doing their job correctly.
Basically we have had vans fitted with fridges in this format for the last 50 years and I suggest that water damage must rate very low on the problem list, unless exacerbated by human error.