if you convert your contours to NURBS curves, you can then use the loft surface…
Novedge Webinar #108: Creating & Editing Site Models in Vectorworks – Q&A with Geoff McBeath
Here is a great blog with some Q&A on site modelling from my friends at Novedge. http://blog.novedge.com/2014/04/novedge-webinar-108-creating-editing-site-models-in-vectorworks-qa-with-geoff-mcbeath.html