Just downloaded the sample and tried to build it, got this error:
(..)Repositories\Unity\Builds-Windows\Vuforia HoloLens Sample\Vuforia HoloLens Sample.csproj(392,9): error MSB3030: Could not copy the file (...)Repositories\Unity\Builds-Windows\Vuforia HoloLens Sample\Plugins\ARM\VuforiaWrapper.dll" because it was not found.
I have succesfuly built an app on the hololens using vuforia before, it worked although the assets were just way too high over the markers. Not sure what is different here...
I am using .Net as the scripting backend and Unity 2017.2 with integrated Vuforia 6.5. Attached is a screenshot of most settings and the error message.