A humidifier or steam from a hot shower(don't put in the shower) can sometimes help. I don't know about the bowl with vicks. Making sure bun is getting the antibiotics is the most important thing as that is what's going to control the respiratory infection.
I would continue to keep a very close eye on breathing. You can see in the links above what it looks like when a rabbit is struggling to breath. They will often raise their head up and will mouth breath. If you see any signs of this or other signs of deterioration, I would get to the vet immediately. With it being the weekend, it would be good to have the name of an emergency vet on hand that treats rabbits just in case.
If bun seems to do well over the weekend and is improving come Monday, I would probably skip the xrays as it's not likely to change the treatment the vet is already giving. If your bun is worse, then yes xrays may be helpful, though I'm not sure it would change treatment any. If your bun worsens or does have pneumonia and increased difficulty breathing, I would want to start nebulizing and oxygen therapy.
I'm glad you were able to get administering meds and critical care worked out. I would just suggest when giving meds, critical care, and water, to go slowly giving your bun plenty of time to chew and swallow, so as to reduce the risks of aspiration occurring, which would be a very bad thing to happen, especially for a bun already having breathing difficulties.