one suggestion is to finish a project in the version of VectorWorks that you started it in. So if this project was started in VectorWorks 8, then finish it in VectorWorks 8. Start all new projects in VectorWorks 12...
however, if you must use this file in vw12, then import it layer by layer, until you find the thing that is causing the crash. Then delete if from the original file in vw8. Then import all the information into a new vw12 file.