The solution is the following (some steps might be redundant):
- Uninstall Vegas 13
- Remove any entries called "Vegas" from the registry with regedit (be careful!)
- Re-install Vegas 13 and register it
- Right click on its icon and modify Target: "C:\Program Files\Sony\Vegas Pro 13.0\vegas130.exe" /NODXGROVEL
- Go to the device manager and disable your graphics card.
- Reboot your computer so it boots into plain VGA mode
- Right click on the Vegas icon and select "Run this program as Windows 7"
- Start Vegas
- Go into settings an disable GPU support in both the general settings and for the 2nd monitor
- Option: If you are adventerous try to re-enable GPU support! ;)