Is the sound healing technique promoted by Jim Donovan to enhance vagal tone effective in doing so, and will it confer health maintenance as well as healing benefits?

This is the case. As is true of all new pioneering efforts, you are hearing the positive side as he is speaking as an advocate. It will not be the be-all and end-all for healing but will have many beneficial uses. It is always good to have the right platform in place to support the self energetically, so this is much like having sound nutrition and sources of sunshine, fresh water, and exercise. It is, in a sense, a compensation for many things that can be inadequate in the life experience and lead to deficit.