...Delila gets sick again, gets better then attacks Bella to "get back at her for attacking her".
Another alternative is to consult an animal behaviourist or animal psychic (some people are gifted and can tune in to an animal's thoughts and may be able to learn what's really bugging Delila and what can be done to change things for the better). I know what you're going through and it's not pleasant especially when you love both cats and things were going well before the vet visit. I wish you all the best, and hope you'll give us an update.
...cats DO NOT have the mental capacity to 'get back' at anyone for something that happened a week ago. That's humanizing them and can cause you to create resentment that just isn't there. If it had happened immediately it still wouldn't be retaliation btw, it would be fighting back, or protecting herself. Cats dont retaliate. We might interpret their actions that way, but that would be us misinterpreting them.
I do agree that one possible solution would be to consult a behaviorist, but I don't think the OP is at that point yet.
I don't know what else I can do, I am really hoping someone has some ideas. I do not think my Vet understands the severity of the situation. Has anyone has any experiences like this that would have any insight?
Things to note:
Anything medical related has been ruled out
I have tried Feliway, Bathing them, vanilla on their noses and Rescue remedy.
I keep them separated at all times.
The last few times I took one of them to the vet I took both of them (except for the first time).
Please, please, please if anyone has any other ideas please let me know! I do not want to get rid of my kitty.
I don't think you'll need to get rid of either cat. I think that considering that after a month would be extreme. Wait a bit, and try some things, then reassess in a while.
First you need to relax. Your stress (personal, about the cats, ect) can actually be making the situation worse. I know with my two girls their issues with eachother escalate if myself or my SO is having a stressful week. So step one is for you to try and settle down a bit. I know that it is a stressful situation, and it's hard not to be stressed when your kitties are so upset with each other, but you have to try.
The next step is to start right from square one with introductions. Check out this link
for a great guide.
I highly suggest that you keep using the feliway. It might not make everything perfectly fine again, but it will help lower their stress levels.
The vanilla was a good idea, but I'd try it differently. One of the cat intro steps from little big cat (the above link) is scent swapping. You take a towel and put it in one cats favorite sleeping spot, and another towel for the sleeping spot of the other cat. After a week you swap towels. Repeat. This is where I would use the vanilla. When you go to swap the towels rub a drop of vanilla onto each towel.
I also feed yummy treats when introducing the scent towel. This will make the association that the other cat's smell means yummy/good stuff. Keep things calm though. Don't talk excitedly ect, you want the cats to associate eachother's smell with good things and being calm and relaxed.
Once that's going well move on to the feeding them on either sides of the door, swapping them into each others spaces, ect. IMO you should go really slow with their reintroduction. Take at least a month for the whole process. Hopefull the slow progress will reduce their stress levels, and yours.
Good luck! I know how stressful this can be. My girls are slowly getting better with each other, but they'll never be best buddies.