A quick check of the internet reveals a 1956 doctoral disseration by Paul
Bryan called the symphonies of Vanhal - specifically:
The Symphonies of Johann Vanhal.
Ph.D., Musicology, University of Michigan, 1956. 2 vols., 524 p.
so if it's a work in progress it's been in progress for a while. More
likely, he set up the system a long time ago, and no one's seen a reason
to change it, and just named it after him.
-Eric Schissel