Hi
I'm no expert, but experience over the past number of years with my own medically compromised cat, has taught me a few things. A lot of my knowledge has come from a myriad of sources.
Your teminology - "non recognition aggression" - fits the bill to a tee. What happens is this. Killer has sensed a change that Kiri went through and the scents that she gave off on entrance to the house when coming back from the dental surgery are different. To Killer, this would be a sign of a new animal into her territory. Kiri would be giving off the scent of being a wounded animal as a result of the surgery, plus the smells from the other animals at the vet's office, plus the smells from the vet office itself. For some unknown reason, cats don't recognize one another visually when put under undue stress, they rely more on scent and what their nose tells them rather than what they see.
Cats are not the same as "pack animals" when put under extreme stress. Dogs and pack animals have three methods of dealing with a threatening confrontation - fight, flight, and submission. With cats it is only the 2 - fight and flight. By the sound of it, the change was so dramatic that it put Killer into the fight mode because there was no place to run, and her territory was being intruded by a strange cat.
Having said that, it could be very difficult to get things back on an even keel. You mentioned that you tried "herbal calming drops" What were they? I've used calming remedies with some success depending on what and how they are administered. You've already done the "separation" bit, and you may have to do it again over a longer period, due to the fact that Killer perhaps doesn't recognize Kiri and feels that there is a "new" cat in the house. If you decide to go this route, please let me know. I can give you some ideas on how to set up a safe room so they can see one another but not interfere with one another.
Something that you might want to try is a Thundershirt. There is some talk of them in the last day or so in one of the blogs regarding them and dogs. They make them for cats as well. I'll include the web site for you to look at. There was some mention of Pet Valu carrying them, but I don't know if they carry them for cats.
Possibly a calming agent from the vet might help both of them. I think I would be using aloe vera juice on Killer to help cool and heal her stomach after all the vomiting. If you go that way, make sure that you get a certified organic preservative free aloe vera juice, Lily of the desert is one thatfits that description.
As far as giving antibiotics to Killer, I'd be very leery of doing that because my epxerience has been that it upsets their stomachs and Killer already has a stomach issue if she is vomiting. Antibiotics also tend to kill off good bacteria as well as the bad.in the digestive system.
Also, the re-action that Killer is exuding could suggest a possible previous bad experience with something. You do not mention at what age you obtained the two kitties, nor under what circumstances you got them. It could have been something associated with a vet office or another environment and it could have triggered a re-call of something very traumatic.
Hopefully some of this information is helpful to you and will trigger responses from other people in the know.
http://www.thundershirtforcats.com/