Thank you for your answer. I would like to understand a bit better why this seems to you to be the spine more than the vagus. If it is the spine, what level would that likely be? Presumably somewhere in the cervical spine? Incidentally, a couple of times when getting off the foam roller, I noticed some joint popping in my upper back, probably right near the end of the thoracic area (between T3 and T1) rather than in the cervical spine. I have loose joints in various parts of my body.
Incidentally, over the last couple of days I have indeed noticed my muscle pains subside after increased hydration.