Yes, I remember that episode and the ring from the Vulcan ship. I ask myself the same question, but I concluded that the Vulcan wouldn't agree that part of there designs would be on the Enterprise, without them designing the rest of the ship...in some fation. From what I know of Vulcans, they are logical but they have a certain "pride".
So I figure the ship would be designed by a team of Vulcans and Humans, and that the result would be an hybrid of their respective technologies.
But that's what I think...that the Vulcans wouldn't let a ship, just be an upgrade. I can't say I am an expert in vulcan psychology tho...