Parasites could cause gaging and regurgitation but there are other things that can cause these symptoms.
Is your vet experienced with reptiles? What testing did he do? Was he able to do a fecal on a stool sample from your dragon? did he do any blood work, x-rays etc?
Intestinal blockage, stomach infections, parasites or other problems can cause vomiting. Respiratory infections could cause labored breathing, bubbly mucus discharge from mouth (don't confuse with normal spit) as well as loss of appetite. If you don't feel your vet is doing the right thing, take your dragon to another reptile vet for a second opinion.
Also, treating for parasites doesn't happen over night, it takes several rounds of treatments to completely clear out the parasites, typically you administer the medication given to you by your vet (give us the name, as some dewormers work better than others) for a week or two then stop, then do it again a couple weeks later. I am not sure, never had to do it myself.
IN the info you posted you didn't indicate the temperatures of your tank nor your feeding/care habits before she became sick.
Below is a couple links to qualified herp vets, find one in your area and give him/her a call.
www.arav.org/USMembers.htm
www.herpvetconnection.com/
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PHLdyPayne