This is a document Preference that tells Vectorworks to save the rendered viewports. The advantage of this is you do not have to update the viewports each time you open the drawing, which could save you a lot of time. The disadvantage is that saving the rendered viewports can make your files much, much larger. In a simple example, turning on the viewport cache made the file grow from 7MB to 12MB.
For Vectorworks manuals, visit: