Wow, that's an obscure streamning video format. The only thing I could find on it indicated that the default video format is the now semi-open-source VP3 (which VLC supports), but that it can use any video or audio format.
If VLC opens the file and plays the audio, that means it knows what to do with the NSV file format, but either doesn't recognize the video codec, doesn't support video playback from that file type, or there's something wrong with the file.
Open up the Get Info pane in VLC and see what it says the video format is--if it is there, and is something VLC doesn't play (VP6, say), that's your reason, and it won't work until VLC or whatever adds support for that format. If no video is listed, perhaps it's an audio-only stream or there's something wrong with it. If the listed video format is something VLC should support, perhaps support for the NSV format just isn't complete yet.
In any of the above cases you're probably out of luck, as between VLC and MPlayer you can play basically any format for which a Mac-compatible codec exists on the Mac, with the sole exception of a couple of Windows Media codecs.