Having signed a contract for a book of a certain link, Vance knows that he has to wind things up at 80,000 words or whatever.

Look at Ports of Call. The publisher had a book of a certain size in mind. When Vance reached that point with only half of his tale told, he just stopped. In the olden days, he would have patched up some kind of conclusion. This time, from a comment in a recent inteview, he seems to have just shrugged and said "what the hell." Onward to Lurulu.