Chickens get respiratory infections, not colds. Unlike cold in humans, respiratory diseases are more severe. They can stay with a flock for its whole life, appearing during times of stress. Or, they might just be a one time occurrence.
I recommend giving them some probiotis and electrolytes, and well as nutritious foods like scrambled eggs, egg yolk, moistened chicken feed, and yogurt. Make sure that the sick birds aren't in drafts. If all of your birds have the disease, there would be not purpose for isolating them, but if only a couple birds are sick, isolate the sick ones.
If the disease is viral, all you can do is give supportive treatment. However, if it gets worse, or doesn't get better, get some antibiotics. You could start out with a broad-spectrum, water soluable one like Oxytetracycline (Terramycin, Tetroxy HCA-280, and Duramycin are all versions of oxytetracycline). But, you could also try a stronger antibiotic like Tylan50. Tylan50 is one of the best antibiotics. It is given as an injection using a small needle (20-22 gauge) once daily into the breast muscle for five days. Give 1cc for large fowl, .5ccs for bantams, and switch sides of the breast muscle to prevent irritation. Remember to not give any dairy products like probiotics or yogurt while giveing antibiotics.
There is a powdered version of Tylan, but it isn't as easy to find. You can find Tylan in most farm supply stores. It is sold for cattle and large livestock, but is commonly used for chickens. Tylan can be given orally, but this doesn't work as well, and I recommend giving injections instead.
If you've never given a chicken an intramuscular injection, here is a link to some information:
http://shilala.homestead.com/injection.html
Here is a chart showing some various medications/treatments/dosages for chickens:
https://sites.google.com/a/poultrypedia.com/poultrypedia/medicine-chart
The VetRx that your are giving them may or may not work. It can help birds recover, but in my experience, it really doesn't do a lot. Still, there is no harm in giving it.
Respiratory diseases will never be fully cured. They will remain with your flock, even if you don't notice it all the time. Sometimes your birds will be healthy, but if they experience a lot of stress, symptoms may reapear. Some people would recommend culling an infected flock. However, many people keep birds that have had a respiratory disease, and their birds never have any problems again and are perfectly healthy.
Hope this helps! Make sure your birds get enough to eat and drink.
