Most Microsoft apps look like they are labeled as device assignable.
Make sure to also select the box under the VPP tab to assign VPP content.
I'm assuming the screen grab above is from JSS 10. We're still running 9.101 hoping to upgrade soon though.
I see this when I "Search VPP Content"

I'm not seeing 'Microsoft OneNote' auto populate under Mobile Device Apps, so I've been adding a new 'App Store app or VPP store app' in this case I search for "Microsoft OneNote" I have 3 TABS at the top VPP Store Apps is blank. It's the "New App" that doesn't allow me to Assign VPP Content.
Upon further investigation I found I was doing something wrong. Originally I was using the App Store on my Mac and copying the URL across (based on another forum post I found with searching issues in VPP).
I just tried doing the same thing but from an iPad. When I search in VPP now I get 3 options where as before I was only getting the one, which I had assumed was correct but was only for the Mac's.
The iOS version now automatically populates in JSS and I have the "Assign VPP Content" option.
Thanks bfrench for providing that little bit more info. pointed me to the solution.