Actual continuing development moved to Naperville, Illinois, in the early 80s, as part of the terminfo stuff.
So the copyright is probably with "AT&T", but since the Unix source code moved to Novell, then SCO, I would assume that the vi source code/rights moved along with it. But that's a guess.
So much for now. Lots more to follow soon. I just need to find the time for this...