Also posted somewhere else but might as well join info here.
After all did a clean install and still only had a black screen in the rift, so by than was thoroughly annoyed as you might imagine.
Solution for me was however surprisingly simple, might help somebody else so will post anyway.
While using the AMD 390 card I had the HDMI port shared with either a monitor or the rift connected. To be able to do so and not have to dive behind my desk and computer each time I had an HDMI extension cable inserted into the video card port and with the end of the extension cable on the top of my desk.
So whenever switching between monitor and rift I had easy access to change the cables from the top of my desk. Worked like a charm while still using the AMD videocard never any issues.
As the 1080 has more output ports I no longer need to switch monitor and rift using the same port. But never realized but still had the rift attached via the HDMI cable extension connected to the nvidia HDMI port.
Removed the HDMI extension cable , directly connected the rift to the 1080 HDMI port and immediately vision popped up.
Disconnected the rift and checked if there somehow was an issue with the HDMI extension by attaching a monitor and that still worked fine. Replaced monitor for the rift vision went black again. So the used HDMI extension cable in combination with nvidia was what kept messing things up while it never was a problem with the AMD card.
Just to be complete the clean windows 10 install was including march 2018 updates and I used most recent nvidia drivers (398.something) .
Rift works like a charm again an by now very happy with the additional VR performance boost from the 1080.