You can add planar objects to a slab using Add Surface. The first step is…
Vectorworks Tip 405 – Splitting A Subdivision Surface
Starting with a subdivision primitive is all very well, but it does limit your ability to create complex shapes. What you need to be able to do is to split your subdivision into parts that allows you to stretch the design into shapes that you want. When use the edit subdivision tool, you will find several modes on the toolbar. One of these modes allows you to split your subdivision. This is what you need to do to create some of these complex shapes.