Hello Charlie - I work more on CT and BoIT than ITP. I do have the question out to some ITP folks. My assumption is because ITP is very particular and TBM Studio is still the reporting surface for ITP outside of canned reports. You would need to send the data to TBM Studio and create he desired view of Projects and Vendors together.
Again, this is my assumption based on my product knowledge and I am still waiting to hear back.
#ApptioforAll