I DO do small dogs and she is utterly adorable. Sounds like you've had a very worrying time, hope she continues to improve. It's extra worrying when they are so tiny xx
So sorry I have only just seen the PM and this thread!! Glad all is ok with her.
Did you get papers with her? What age was she supposed to be? She seems, from the photos posted, to be very very young. Chi pups shouldn't go to new homes until at least 12 weeks (very small ones should be 16weeks+) and are usually still with mum until about 8 weeks as they are slower than other breeds to wean and to develop. Without seeing the pup at that time it would have been impossible to tell, but she looked very young to me, less than 8 weeks so that could be the problem. Young chi's are prone to having blood sugar crashes like your vet found. I have had this happen once before with a pup of about 8 weeks, pup was nearly dead (literally flat out unable to stand and not responding) and the vet gave her fluids immediately which helped her, but wanted to put it to sleep. We decided to ring the breeder we bought our champion dog from, a very experienced breeder in America who advised that it was low blood sugar. We fed the puppy honey and milk formula regularly throughout the day and within a few days she was completely back to normal.
Such a small thing but can very easily kill these small breed dogs.
Re. vaccinations, I am no longer breeding Chihuahuas but when I did I vaccinated at 10 weeks and 12 weeks, before they went to their new homes. My other breeds I vaccinate at 8 and 10 weeks. I do the one booster vaccination at a year and then no further vaccinating.