Seems reasonable. Alternative approach might be to model the area of the vault with a different soil of very low modulus so the surrounding stiffer soils absorb the load.

But is this how it will really act in practice if you don't do something to isolate the vault from deflections in the footing?

I believe it will apply the default soil properties in areas without specifically assigned soil regions. You could create a soil definition with a Subgrade Modulus of 0 and apply it in this area to ensure that the model will behave in this way.

I spoke to a co-worker about this and he recommended what you all are recommending. He said when he didn't apply a soil region that looking at the results of the nodes, it still produced pressures at those nodes. So, I've created a new soil region and set the subgrade modulus to 0, and the bearing pressure to a very small value as Risa spit out an error for setting the bearing pressure to 0.

I would prefer to define a soil property with essentially zero stiffness (but a reasonable bearing capacity compared to the regular soil) and apply it to the region where you have the cable vault. The reason is because, I believe RISAFoundation has "default" soil properties that it will use if you don't assign something. Those default values should be defined in the (Global) Model Settings. The reason why I give it some reasonable soil capacity (maybe 1/10th of your regular allowable) is because I don't want to see accidental failures.

Note: one other thing to look out for is that the mesh divides at the locations where your value occurs. You might even consider having multiple slabs (even if they have the same properties) that connect along a common edge. That gives you a little more control over how the program meshes.