I am so sorry to hear about your loss. Do not second guess yourself. You loved your pet right up to the end and should not feel guilty.
In my experience, some dogs do recover from vestibular disease but these dogs are usually young and healthy. Most dogs never fully recover but they may improve with time.
Vestibular disease produces stroke-like symptoms and may cause the dog to walk in circles, hold the head at a funny angle and lose control over one side of the body.
I have a 15 year old rescue dog here who suffered the effects of it 2 years ago. She still shows the symptoms of it and I doubt myself every day for not humanely euthanizing her at that time.
I think that you did the right thing. You went about it the right way, discussed it with and had it diagnosed by your vet. You then made a choice based on what he told you.
Again, I am sorry for your loss. If you don't know about the rainbow bridge, please let me know and I'll share the story with you. You might take some comfort in it.
Let me know if you still have questions.