Is high vagal tone important for regulating blood pressure, digestion, blood sugar levels, inflammation, brain function, even libido, and also slowing cellular aging?

This is all correct. It is not the only influence. There are many ways these phenomena are regulated and perturbed depending on the nature of the influence one teases apart. As you know, physiology is quite complex, some physiologic changes have relatively few major operating mechanisms but others have a quite extensive series of potential influences that will have some consequence, however incremental, and then there is an interplay of multiple forces resulting in an overall net effect. Vagal tone is much like the latter, it is a modulator of energy in a way that impinges on multiple types of cellular functions and the interplay of many mediators, both chemical and electrical, orchestrates the consequences you describe, so there are many interesting opportunities here to make changes and to enhance well-being and help to return disturbances created through chronic illness to a normal setpoint. It is very much a kind of energetic reset that it achieves and thus will have great value as a sort of tonic when used on a regular basis.