Grabbing frames in memory from the C API

[Rangifer]

I'm doing some personal experiments using libvlc with C# interop, in order to learn more about different media libraries (I have worked as a developer with DirectShow). I have checked out the existing .Net wrappers but none of them seem to do what I'd like to do, which is grabbing a few uncompressed frames and turn them into in-memory GDI bitmaps.
So far I wrote p/invoke calls for a few libvlc_* and mediacontrol_* functions and they work, I managed to load and play a file in a window. mediacontrol_snapshot() however returns null along with a "No video output" exception, even though the video is actually displayed. I seem to understand it's not the function I need anyway.
Various forum threads suggest that uncompressed video can be grabbed using the "vmem" plugin for video output, but I cannot figure out how to select/access it using the C API. Is there any example of how to do that?

Thank you for any help.