I'm afraid I don't think they have much idea at all about the proper treatment for Dot - being hyper gives some support to failing kidneys, and reversing it so quickly risks causing total kidney failure - to overdose her like that shows a very high degree of ignorance of the condition, in my opinion
if you posted her levels on the Yahoo group they would be begging you not to medicate at all until the level had come up, and certainly not at 5mg a day - I don't feel I have their expertise to say that myself, as I understand it is so difficult to go against your vet
lethargy is the first sign of overdose, but at Dot's age she isn't going to be particularly lively, so it's a difficult one - go by your gut instinct, I'd say, whatever anyone, including the vet, says - keeping her as well as possible is the main aim, and you are the best placed to achieve it
I haven't researched the difference in the medications - I think they are based on the same drug, but Vidalta was formulated for animals, and Felimazole, which is the same as the Methimazole used to treat overactive thyroid in humans, was licensed later for veterinary use, which is why some older vets stick with the Vidalta they first started to prescribe