After installing Vegas Pro 13 on my new laptop (Dell Precision) Vegas properly identifies the two Dell monitors and allows me to choose display 2. However, which clicking the monitor icon in the preview pane, the preview splits both displays, overlapping the Vegas session. Pressing Ctrl+ALT+F4 does not change the display. Running Windows 10.
Make sure you have the latest graphics drivers (and everything else) from the Dell support page for your model. Or you could try the latest graphics drivers from the NVIDIA site, which may be more recent but not yet certified by Dell.
The issue turned out to be with my new port replicator. I powered off the laptop plugged into the replicator then powered back up. All is good in the Vegas world now.