Slabs are a hybrid object, which means that they have both 2D and 3D components.…
Vectorworks Tip 384 – Add Surface with Slabs
You can add planar objects to a slab using Add Surface. The first step is to create your slab object. Next, draw your planar object. In the images above of use the automatic working plane to draw the planar object on the same plane as slab. Select the slab and the planar object. Right click and choose Add Surface. This will add your planar object to your slab.