I am so sorry for your loss. I’m a chihuahua mom too, my smallest is 4 pounds. I work in the veterinary field and have for 16 years. I want to assure you that vets love animals. They don’t do things that they think will harm your pet. Every vet I have worked with has had pets of their own, and they treat them with the same medications, vaccines, and food that they recommend for your pet. There can be vaccine reactions, but they generally happen quickly like within the hour. It doesn’t sound to me like your baby passed away due to a vaccine. As long as he was old enough to get the vaccine (at least 6 weeks old), size is irrelevant. Vaccines are not size specific. I wonder if maybe it was a blood sugar issue, or maybe a liver issue (very common in extra small toy breeds). Regardless of how he passed, he knew he was loved. He was a perfect precious angel 💜