Minky sorry for the loss of you your beautiful babes.
Firstly I would be getting another Vet.
I would be looking into the vaccine used and the brand and keeping an eye out on the batch number even contacting the manufacturer and reporting your deaths. My Vet tells me there are vaccines and there are other vaccines.
Checking further into the reaction Tibetans have to vaccination as well.
After vaccination of a C3 you can have what the vets refer to as a false positive to a parvo test which is usually done from a swab from the anus or a fresh stool, anytime after 48 hours post vaccination.
I personally don't believe your puppies have passed because of parvo, but before you vaccinate anymore you need some answers.