I just upgraded to Vegas Movie Studio Platinum 15 (Build 116) on both my laptops. External monitoring on Platinum 15 does not work on either machine. I have both Platinum 14 and 15 on the Lenovo machine and external monitoring works fine on 14 but does not work 15. On both machines the external monitor window goes black when you hit the External Monitor Icon button, the machines do not freeze up.

I can undock the preview window and drag it over to the external monitor and that works fine as a work around for now. Both machines are built for gaming and have dedicated Nvidia GPU's. Running the Windows 10 Home with latest updates.