To the best of my knowledge, it's theoretically possible to basically mature a human body including a potentially-functional brain, while keeping that brain in a vegetative state the entire time.

I don't think this is currently possible. The body just wouldn't work. A large part of the 'wiring' during infant and childhood is connecting body parts and functions with higher and higher level concepts. Think about toilet training. You aren't even aware of how it works but it nonetheless somehow connects large scale planning (how urgent is it, when and where are toilets) to the actual control of the organs. Considering how differnt minds (including the connection to the body) are I think the minimum requirement (short of signularity-level interventions) is an identical twin.

That said I think the existing techniques for transferring motion from one brain to another combined with advanced hypnosis and drugs could conceivably developed to a point where it is possible to transfer noticable parts of your identity over to another body - at least over an extended period of time where the new brain 'learn' to be you. To also transfer memory is camparably easy. Whether the result can be called 'you' or is sufficiently alike to you is another question.