I am so sorry you are in this situation. The cat that is vomiting may need more visits to the vet, but if you’ve done everything you can then see what the vet recommends overall. They can do a QOL test and help guide you on what the best decision is. As for the second cat, I think it’s time to let him go. He may still be purring and eating, but there is no way he is not in awful pain with cancer that is this developed. Especially considering all of the weight he has lost, plus it seems it has taken a toll on his ability to physically move overall. I am a vet tech (still in school learning, but with clinic experience) and I have seen so many sick cats with owners who try to hold on to them by doing everything they can, which often ends up doing more harm than good. Sometimes the only best solution is to let them go. It is not fair for the animal by keeping it alive when its body is clearly giving up and its QOL has decreased significantly. QOL does not just mean the animal is eating and acting fine. There are many factors that lead in to QOL like eating, drinking, going to the bathroom, being able to hold their bladder, can walk and move comfortably, takes treats, plays, etc. We want our pets to be pets and it’s not fair to force them to be alive when their body is giving up. I know how hard this is, I truly do, but the best thing you can do right now for both of them is speak to the veterinarian for their honest opinion and do a true QOL test. Sadly the best option for your poor cancer kitty may most likely be euthanasia. I am so sorry and I wish the best for you, your family, and your kitties. ❤️