Whenever I press the external monitor icon my second monitor shows a screen but it's blank. Vegas 15 pro
And when you move the playhead on your timeline, it stays blank?
Not sure at the moment what it was exactly, but sometime in the past I had some problem too with secondary Windows display. That time the problem was my .Net version, and it was still on Windows 7.
I had to install .Net 4.7 and it solved - that's sure.
First check if you have the latest build of Vegas15. (build 416)
Then check what version of .Net do you have installed.