I'm so sorry about your pup.
It could've been so many things with those symptoms. It may have been hypo as MyFairLacy said or possibly something like liver shunt or hydrocephalus.
If the pup had a distemper vaccine on the 3rd, the vet shouldn't have told you to get another one until 3-4 weeks later. Food and water also can't be withheld form a Yorkie pup for that long. Do you have a system there for reporting vet malpractice?