If you have a set of 2D polygons that represent contours, you can use the…
Vectorworks Tip #212 – Site Modelling – 2D Polys to 3D Loci
The 2D Polys to 3D Contours command can also be used to create 3D loci, which can also be used to create your site model. Use this command where the polygons have regular elevation spacings.