Venice really has an interesting history. I came across a lot of stuff in the bay linked to Venice. I bet you would have a hell of a time visiting the area, as Venice left a lot behind them.
I'm not too sure about Cattaro in Vanilla, I very rarely play vanilla, but knowing that its got its own province in both VeF and M&T shows it should be, its also an incredibly important holding for Venice fought over on many occasions, especially once Dubrovnik becomes an Ottoman protectorate.
Well, that was the whole point. But not only because Cattaro. Bay of Kotor covers a relatively small overall surface, but the place was of huge strategic importance, partly because there were several towns in very close proximity - etc. All of these towns were well fortified - especially Castelnuovo, which overlooked the entrance of the bay, and Cattaro, which was the trading center, deep inside the bay. Before the channel of Kotor, in the bottleneck area is a place called Verige - where the traps would be raised if the invading fleet would head towards Kotor.
As you pointed out, Cattaro with all it's sister towns, was a very important factor in Ragusa's trade sphere, often rivaling and hindering their trade prowess. Durazzo was a very important city in Venetian control, but i find the Bay of Kotor [Cattaro (Kotor), Herceg Novi (Castelnuovo), Perast (Perasto), Risan (Risano), Tivat (Teodo)] along with Montenegrin coastline towns [Budva (Budua), Bar (Antivari), Ulcinj (Dulcigno)] is way more important turf than the city of Durazzo, and having Durazzo lie there in vanilla just make me want this province even more. At this current state, the Montenegro area is underrepresented - indeed the inner Zeta (excluding the Venetian occupied coastline) was of little importance due to its hostile terrain, small towns and small population, but towns in Bay of Kotor were a complete contrast - they had huge cultural, commercial and military importance since time immemorial. Kotor was actually the main trading port of Serbia, even during the imperial era, and our rulers hired administrators from Kotor as their treasurers. The trading importance of this area should never be overlooked like that, and all these towns make this province a very viable one, so much as to give it a trade modifier. Dubrovnik is a 30 minute ride west of Herceg-Novi! Furthermore, the province doesn't have to be limited to just the area of the bay. Venice controlled the majority of Montenegro's coastline in 1444! The idiocy of Đurađ II Balšić selling possessions to Venice led to consequent straight up conquests of nearby towns. If you didn't read about it,
First and
Second Scutari War are a nice read!