This can happen if another program has become associated with Inventor assembly files. In this situation, the message was caused by importing an Inventor file into Autodesk Showcase 2012, which although is not a qualifying CAD application for Vault, it is still a part of the Product Design Suite.
The workaround is to close Vault Explorer and Inventor and re-start Inventor. This will re-register Inventor with the Operating System.
This is an extract from the Cracking the Vault, Manufacturing Community Blog.