You can set a border for a site model easily in VectorWorks 12.5 and 2008.Create…
Vectorworks Tip 310 – Setting the Maximum/Minimum Values on a Site Model
The settings for the Maximum and Minimum elevations allow you to choose how much of the site model you see. Think of these settings as a filter, choosing how much of the site model you want to see. For example, you might have a site model that goes from sea level to 100m, but the building site is around 85m. You could filter the site model from 65m-95m, so that you only see the important parts.