This could be related with the navigation graphics issue that was around and most of the time was caused by compatibility issues. Now this was patched up with service packs.
Please give it a shot and go to Tools-Options-Vectorworks preferences. Open the Display tab and change the navigation graphics either from "Best performance" to "Good performance and Compatibility" or the other way around. If this will fix the issue don't worry working on the selected option this will most likley just improve the performance of Vectorworks.
This issue was fixed in Vectorworks a couple of Service pack's ago so to further look into this and to make sure that all navigation graphics modes would be working please update your Vectorworks to the latest service pack. If that is already done and you will still see issues, don't hesitate to contact the local distributor for further troubleshooting. Again just to remind you that there is should be absolutely no problems working on a different navigation graphics mode.