The Vet should have warned you about the vomiting. Not all cats do, but most will. The first shot makes them nauseous. When Dax was put down, she had muscle contractions which indicated she was going to vomit, but she did not.
I was petting Dax as she got the first shot and looking in her eyes. I tried my best to hold it together for her and keep her calm. The Vet warned us it might happen, before the shot. Dax was at a point where she could no longer move, so it was still better than the alternative for her. Sorry for your loss and it is sad you were not warned about what was going to happen, but it sounds like you made the right choice for your cat.
Smokey (1959-1959, car), Prissy (1966-1968, car), Tina (1955-1974, old age), Rags (1976-1980, stolen), Dax (1999-2015, my choice, due to renal failure), Shelby & Jag (2015) You only think you are training them. When they are gone, you finally get it.