Slabs are a hybrid object, which means that they have both 2D and 3D components.…
Vectorworks Tips 448 – Connecting Walls and Slabs
When you create walls and slabs, you can connect the wall components to slab components. The first step is to set up a wall style with components. One of those components should be a core (I usually use the structural part of the wall for this).
When you create the slab, you can bind slab components to wall parts. You can’t bind to all of the wall components, but you can bind to the inner component, outer component, and core.