You can use a planar object to clip a portion away from a slab. If…
Vectorworks Tip 399 – Editing Slabs That Have Used Clip Surface
When you edit a slab that has been modified, you have the option to edit the modifers. This means that you can change the modifiers to suit the change in the design. But be careful, you can only edit the modifier that is already there and you are not able to add any other modifiers. This also includes substantially changing the modifier that is already there by using add surface or clip surface on it.