I've been running vans for 8 years on their payloads first thing every morning. What's the problem? These van get given a payload and are designed accordingly. So running it carrying a weight it's been designed to carry, where's the issue? It's ok I don't expect any answer as there isn't one!
The answer boils down to how manufacturers come to their 'quoted payload'. Which in the case of small vans anyway, simply means the gvw minus the dry weight of the van. No fluids etc, in fact going by my weighbridge encounter, i am not even sure there it takes into account things like interior trim and seats!. If you get stopped by vosa, i am not sure how you might fare by using the argument that the brochure says whatever kg's, and i am carrying less. In theory you might get away with that, but only once. It is your responsibility to ensure you are not exceeding your van's gvw, and therefore, the limitations on brakes, suspension, bearings, clutch life etc.
I went over a customers weighbridge with a full 500l tank in my van with (900kg payload) and was 40kg under the gvw (and no, i don't carry heaps of other stuff), so you will have to excuse my skepticism on the matter!.
But unless you have been over a weighbridge with a full tank of water, and the van set up as you would for working, you just don't know if you are breaking the law.
Easy clean, go on a weighbridge with your full 500l tanks, and find out what the weight is. If the vans are overweight, and i strongly suspect they are, then that means you are expecting your employees break the law and put their licenses at risk at the start of the day.