Here’s a fix for users of V-Ray for Maya who are having problems with V-Ray’s VFB disappearing. I’ve seen it happen with some older scenes and the fix is simple: open your scene as a Maya ASCII in a text editor and delete the VFB parameters:
Search for “VFB” and delete that set of data. Save and reopen in Maya and the VFB will work again. These .ma text files can huge so I recommend BBEdit if you’re on a Mac. It’s very fast with monstrous text files and has a MEL syntax highlighting plug-in if you write MEL scripts.
If you have a file that is in Maya binary format and can’t save out as Maya ASCII because of the “unknown data” warning, run File/Optimize Scene Size to remove those Unknown nodes.