Hi! I had that same problem and I lost my nerves many times trying to figure out way to take screenshot that would work.
I think that this isn´t the best way to do it ( I dont use VLC at all), but it´s worth of try
I used the VirtualDubMod to open the videofile (I dont know what kind of video file you´r trying to play, but lot of them work in this program), and runned it to the spot that i wanted to take the snapshot and then i just went to Video, and Snapshot source frame or output frame (I dont remember which it was) and saved it at bmb format and had a nice little snapshot of the video.
This is just my way to do it, but it worked for me