If you still have the prefab for the complete avi, you can utilize Unity's FBX Export plugin to try and export an fbx out of the prefab and go from there. Which the defaults for that should be set to work with Blender already, but if not, then you can always look up what version Blender uses out of the two. (I believe it should be ascii? Could be wrong, I always forget lol.)
Anyways though, you should be able to make an FBX out of an already existing prefab from the VRCA you exported presumably. Though if it doesn't export to blender properly, aka has screwed up weight paints and blendshapes... Then chances are that the model has been ripped quite a few times and uploaded, winding up with a version that you have that has probably been ripped maybe a dozen or more times by this point. But if it's the first rip of a model then you'll be near guaranteed to have a fully functioning almost fbx for importing to blender.
