To quote someone else, "what vestigial stage?" ;)
In certain single-celled organisms, the bacterial flagella - Dr. Behe's favorite example, no less - is clearly related to the Type-III secretory mechanism found in certain other single-celled organisms. One is pretty clearly descended from the other, and yet both are completely and fully functional, albeit with different functions, with no hint of anything "vestigial" about either one.
You have simply inserted your own requirement of "vestigiality" into the notion of "transitional forms" - one that is neither required by the theory of evolution, nor is it necessarily observed within the evidence itself.