I have an integrated ATI GPU on my mobo that caused the problem. The standalone PCI-E card is active in the UEFI - there's no option to disable integrated one only to choose an active. Because of this, the device Manager in Windows is still listing both cards and unfortunately Win10 installer installed the ATI drivers automatically for the integrated gpu aswell.
It wasn't enough to Uninstall the integrated gpu in the Device Manager, because Windows automatically reinstalls the device over and over for some unknown reason so I had to Disable the mobo gpu completely (in the Device Manager) and voila, Vegas loads up with no problem.
Because I was able to install and use Vegas for years in Windows 7 with the same rig, this might be a conflict when occurs if you have an ATI chipset mobo with an NVIDIA graphic card, caused by the forced stupidity of Windows 10, I believe.
I had a similar problem starting Vegas Pro 12 under Windows 10 (and which ran well under Windows 8.1). I found that the problem was a conflict between two graphics cards installed: AMD R7 M270 and Intel HD Graphics Family. I disabled the Intel card in Device Settings (leaving the AMD card enabled) and Vegas starts up well.
Thanks for pointing me in the correct direction to resolve the issue.