Mike, did you read the article from Cornell that Sharon linked to? It speaks to the excellent results from using doxy for dogs and also about the various tests for determining antibodies -- the very things you question.
It also mentions the risks/benefits of the vaccine that your vet is selling. It may be making your vet rich but not mine. Even after telling him that we visit our dogs breeder in PA who lives on a deer rich game preserve, he did not recommend the vaccine.