Hi, K D.
I'm so sorry to hear that Nicky has all these upper respiratory symptoms and a cough and your vets don't seem to be able to help him. It's possible that he's not just suffering from a URI and/or allergies, he may also have the feline herpes virus (not related to the human herpes virus at all). Here is more information:
You can use L-lysine products made just for cats, to help him with his symptoms and in addition, don't bring him into the bathroom when you're running a hot shower anymore, as in an older cat of this age, it might strain his heart too much, but definitely continue with the humidifier/vaporizer in any room he's in, to help with congestion. Do not use Vicks anywhere near him. I'm glad you realized it was not good for him before it was too late, but don't put any in the vaporizer, don't put any on him and don't let him smell it or lick it---it's very toxic to cats!
Here is information about L-lysine for cats:
Although the coughing seems related to the congestion with mucus, coughing can also be a symptom of other conditions. Heart conditions like cardiomyopathy and heartworm disease can cause coughing, so make sure Nicky is checked for these conditions.
Since this is going on for so long, it would be advisable to have Nicky evaluated by a veterinary respiratory specialist. Specialists are typically found in 24 hr. ER vet centers. Especially if his vet is away for 2 more months, he needs to see another vet until she returns. You usually need a referral from your regular vet, to see a specialist, but, if your vet is away, you said you brought him to another vet in the past, too, so if needed, get a referral from that other vet.
I hope all will be well with your Nicky!
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