Hi, I wanted also to post about that as I have the exact same problem. This looks like a bad bug. I have a RAdeon-Fury X and the "external preview" work with my 2 monitors in V13. I am quite new to Vegas as I have been taking the special offer at 199$ and so I am still waiting to get my V14 serial.
Desktop here running Windows 7, with two monitors, works fine with 14 as it does with 13. Using 'Windows Graphic Card', 'Display Adapter' showing 1 and 2. R9 380 with one connected to DVI-I and the other to HDMI.
On my desktop(Windows 7) the similar problem: video output selection dialog box is empty. Video adapter: AMD Radeon HD 6670. I tried different versions of drivers (Catalyst 13,14,15) but the the same result.
Thanks vitus. So a few of us seeing this with diffrent hardware and drivers.. I believe if no secondary display is detected for whatever reason by Vegas, the option is not displayed in the dialog box. But it should be getting detected if the setup works with other versions.
wrote on 9/21/2016, 3:42 PM
When I upgraded to the latest edition of Windows 10, I had to reinstall my drivers, even though they were still there. Have you tried that? I know you didn't install Windows, but was just a thought.