I had not been able to replicate this until I started working with a new Windows 8.1 machine, where Vectorworks does not close naturally. However on other 8.1 machines here it closes without issue.
Before that in Vectorworks 12.5.3 through Vectorworks 2014 SP2 I did not see the problem on Windows XP, windows 7, or OS X 10.6 - 10.9.
We are still trying to figure out where the problem might be, since it doesn't seem to be OS or hardware dependent. If anyone encounters this problem, please send tech@vectorworks.net one of these from the affected machine along with a description of what version of Vectorworks you are using and how frequently it is happening:
http://kbase.vectorworks.net/questions/627/DirectX+Diagnostics+and+System+Profiler