If your hen has a respiratory virus, especially if they are "vaccinated," in my opinion, the worst thing you could do, is put it back in with the others. Follow your instincts. You know better than any of us even if you don't think so right now. Integrating a hen who lost thier place in the pecking order is way easier than killing your whole flock. As chicken owners, we bring in new hens all of time, and they integrate and figure it out. I am just trying to help, please disgregard if anything I told you sounds too scary and wrong information. God Bless you and please update the status of your flock.