Regarding the old man's immediate linking of the vase and the morphine clinic: He already knew that Veronika was a client of Dr. Katz as a result of the conversation he and his wife had with Robert early in the film, when Robert first approached the doctor's building thinking Veronika lived there.
However, I suspect that the man said what he did when selling the replacement vase because the original - and perhaps much of the boxed-up stuff in Veronika's house - actually belonged to Dr. Katz, who had put it in the house in preparation for her move into it.
Remember that scene in which the doctor refers to the house as her own, and Veronika retorts something like "not yet, I'm still alive?" Veronika says that she can pay for each dose with her jewelry, which suggests she doesn't have a lot of other property to trade away for morphine.
Likewise, when she meets Robert in the hotel cafe at the beginning of the movie, she asks him for 300 marks (at that time, only about $70 U.S.), allegedly for a brooch. She picks one at random, returns it promptly, and desperately asks for her money back, presumably so she can buy her next fix with it. This led me to believe that she isn't attached to her possessions, most of which she's willed away already; she's always got an angle for how to turn them into a potential fix.
Anyway, that's why I suspect the vase was actually Dr. Katz's. It would explain Veronika's concern over replacing it immediately and surreptitiously as well as the old man's immediate recognition of it as linked to Katz.
As for the elderly couple's suicide, I figured it was accomplished by stockpiling drugs; it doesn't really matter what kind. In that first scene when Robert runs into them outside Dr. Katz's office, their droll comments (about how that kind of doctor must be harboring secrets for a lot of people) seem to betray some clear-headed distance from an addict's mindset.
However, I'm stumped as to why they'd have left their estate to Dr. Katz if they weren't truly dependent on her for morphine. And I didn't really understand, either, that brief visit the couple made to Dr. Katz's office toward the end when they were hurried away. Do you have any ideas?
No, sorry I don't have any ideas about that. I saw VV a few months ago. I think there are some aspects to the elderly couple's 'situation' in the film that are, perhaps, unintentionally ambiguous, although I agree with you that some things regarding them do have explanations. Those few unresolved questions seem to me to be the film's only flaw. Certainly it is Fassbinder's most stunning black-and-white vision.
The wife makes a comment about the husband going through a lot or suffering for the last 10 years -- i.e. since the war and the Holocaust ended. I assumed that she meant he is suffering serious psychological trauma from his experience in the Holocaust. Dr. Katz's opiates help him numb the pain. He may not be addicted in the same physical way that Veronika is, but he may think he needs the morphine to get along.
Then Dr. Katz decides she's done with him. Enough toying around. She wants to be rid of the old couple, or maybe it's simply time for her to get the house. So, she cuts off his supply of morphine. Unable to deal with his memories or nightmares, he decides to commit suicide, and his wife joins him. They did it by taking sleeping pills that they had been hoarding over time. Remember, the sleeping pills have a bitter taste. That's why they take them with tea and honey.
Another odd think about this couple is their names. When they first meet Robert, outside Dr. Katz's building, we learn their first names are Jan and Jenny. Later (at the time of their deaths, if not earlier in Dr. Katz's waiting room), we learn their last name is Treibel. "Mrs. Jenny Treibel" (in German, Frau Jenny Treibel) is the name of a famous German novel from the 19th century.
I don't necessarily disagree w/ you but I think maybe you are reading between the lines to some extent. But thank you for your thoughts on the subject.