I would not use either eucalyptus oil or Vicks. The principal ingredient of eucalyptus oil is cineole. Cineole, which is a part of the chemicals family that is a strong reptile irritant. I recognize this is out of a limb a bit but Cineole is a component found in rosemary oil as well. Generally (really general here) any ol ending chemicals are bad for reptiles. (phenol and any and all phenol variates.) It is certainly a huge question mark for nebulization.

http://digitalcommons.unl.edu/cgi/vi...%20reptiles%22
(I have done a lot of research about the brown snake)

I have never heard of tylan as a treatment for respiratory issues. Reptiles react to drugs differently than other animals and much much slower. I know tylan as a digestive treatment I am unaware of it as a respiratory treatment. I might PM skiploader and ask an opinion but to the best of my understanding it is a unknown.

The best action is a culture and treatment based on the results of that. You may have the expertise to do a lung wash yourself and send the culture out to a lab. i certainly don't and would never suggest that with out a great deal of experience.