I have the same problem as I reported in another thread. I tried the trial version of ArkMC on my MacBook Air and that is able to see the UPnP servers on my NAS. It seems that VLC is unable to replicate what ArkMC can do.
Well, VLC certainly does so on IOS and other platforms. I dont think this is a case of "unable," Im sure there's a reason, it may be that it's not a quick/easy fix or they have other more pressing issues to deal with at the moment.
Right now as a workaround I am using the bonjour network discovery which finds the server and connects via FTP - doing it that way works, It's not as fast or as elegant as it is on IOS or other platforms but as home media NAS's are becoming more widely used im sure eventually they'll find a way - it's becoming a more important feature by the day.
Im sure there are other applications, but I like VLC - it really does everything and is stable and tested, so im going to donate and have some faith that they'll work it out eventually.