So glad that things are better!
While I understand not wanting to let Hermione (great name btw) drink so much that she vomits it back up - make sure that you aren't limiting her water too much. If she does have any kidney issues, that can make the problem worse. A bloodtest is a very good idea, just to see where everything is at. It'll also give you a baseline to see any future changes since you don't have any medical history on her.
Bless you for adopting an elderly dog - and I agree that your son is awesome for wanting one!
Livin in a Newfie Drool Zone