A ring ship wouldn't make sense for the plot. Why would a solely human made engine use a vulcan design?
Why wouldn't it? Vehicle design is shaped by physics and engineering principles, form following function. Logically, most cultures' designs for a given type of vehicle should converge upon certain fundamentals. The ships of China's Ming Dynasty treasure fleets were much huger and more elaborate than the tiny caravels the Europeans used at the time, but they were both equally recognizable as sailing ships, because there are certain physical necessities a sailing ship needs to have. Now, maybe there can be two or more feasible designs for a particular type of craft -- catamaran vs. single-hull boats, fixed-wing vs. rotary-wing aircraft vs. airships -- but there would realistically be a finite number of configurations that would work for an FTL starship, and different civilizations would either converge upon them independently or adopt them from each other because they work.
The way I've explained the ringship in my Trek novels is that it was a prototype for a Vulcan-inspired warp drive that was developed in competition with Cochrane & Archer's nacelle-based drive program. I went with the explanation in the
Ship of the Line calendar foldout that the ringship type of drive wasn't as maneuverable, so humans ended up preferring the nacelle approach.
That doesn't explain why Earth decided to mostly go with saucers, though.
I addressed that in my first
Rise of the Federation book. Circular or spherical shapes are the most compact configurations of a given volume, minimizing the greatest distance that a person or a supply conduit has to cover to reach every part of the volume, which makes them more efficient. For combat-oriented ships, like the Vulcan and Andorian craft of the ENT era, a long, narrow configuration is good because it minimizes your forward profile and makes it harder for an enemy to hit you. But for exploration and multipurpose craft like Earth ships, or later Federation Starfleet ships, a wide, round configuration gives you more efficiency and versatility in using the interior space, which is good for that kind of ship.
In free fall, a sphere would probably be better than a saucer because it's the ultimate in compactness, requiring the least material to contain a given volume and thus enabling the ships to be lighter and more economical. But with artificial gravity, I suppose it makes sense to go with a flatter, but still circular, design, so that you don't have to climb as far to get to the top of the ship.