Use the white vinegar and water, soak the carpet and pad well, and use a rug cleaner with vinegar instead of water in the tank with the shampoo. Uses the rug machine to dry it as much as possible, and move the litter box to a vinyl or other hard surface with a good, solid floor mat under it. I would use a big nylon, with rubber base mat under anything like a litter box in the future because it also helps with tracking litter, and gives a better grip too. The ones I'm talking about are rubber backed, very dense nylon pile, and I think should be with the door mats. They're something like 4 x 6 or 3x 4 and will help in the future.
You might have to get a commercial rug company like Stanley Steamer to do it too.
The product I use the most successfully is Natures Miracle. Seems to be available at most pet stores. Soak the area in Nature's Miracle and put a towel over the area and a rubber backed carpet over the towel. The object is to keep it wet for up to a week. That has worked the best for me and I have two cats and a dog. One of my cat's marks when stressed. Good luck!
I forgot to say the part about standing on a towel (or even better those ShamWow towels) until no water comes up.
I think someone on here said they changed the Nature's Miracle formula a while ago (or the company changed hands and then it changed) and it's now under another name.
I agree with Nezzy though. Even if you get the smell out of the rug, and even change the padding under it, when it gets wet the smell will come back, especially after the number of times the cat used the rug. So when you move and the carpet cleaning company comes the landlord will know what happened and charge accordingly.
you will never get the smell of cat pee out of carpet. Sorry.
If you're lucky, the carpet is on a slab. That way only the carpet and pad must be replaced.
A good friend spent much $$$$ replacing the wood subfloors in his home, two years after he bought it from the previous cat-owning family. Any time there was rain or prolonged humidity, the house stank of cat urine. He tried everything, but in the end all he could do was replace the subfloor.
Perhaps you can mask the odor long enough to get the security deposit back! If not, I'd be taking some of that money out of my roommate.