This is a Document Preference that tells Vectorworks to save the rendered viewports. The advantage…
Vectorworks Tip #154 – Document Preferences – Save Viewport Cache
When you open a file that has viewports in it, you will have to update those viewports if you do not turn on Save Viewport Cache. This can save you a lot of time waiting for viewports to regenerate, especially if you have a lot of rendered or section viewports. However, the downside for this is that your file sizes will be huge compared to the files that do not save the viewport cache.