Which side of the VirtualBox are you running iTunes on? (Which OS will you be under running the iPhone sync)...
I use Parallels to run MC17 as a library client, and it works great.
I currently use MC17 on the library server to do what you've done in the past (maintain a separate library for my Macintosh--the iTunes library local on the Mac, the files on my NAS). I sync my iPhone with this library on the Mac.
However, I ALSO maintain an iTunes library on my main MC Library Server machine--using MCiTS (see the 3rd Party Board for much more on MC/iTunes Synchronizer). I'm evaluating whether the use of iCloud will allow me to make my iPhone more platform agnostic--where I can maintain my 'info' on the Mac with the tools I know, yet sync my media off the Windows machine to cut some steps out of my toolchain.... (eg. I'll use MC for playback, tagging, etc.; use MCiTS to mirror the library on the windows machine; use Windows iTunes to perform the iPhone sync.)
I don't know VirtualBox, but perhaps your syncing will work better if you have the "Play local files" option enabled on your MC Library Client AND you make sure you have your network drive(s) fully mapped to VirtualBox? (I use mapped drives in Parallels instead of UNC paths)....?
brad