Even while still sick they will start to perk up as their strength returns when fighting an infection. How long did the vet tell you to continue meds? As you get closer to the end of the meds you will or should see a great improvement in appearance, food intake, ability to keep foods down, etc. If even as meds are about to run out your bird is still having the lethargic or puffed up appearance or vomiting (usually they use the regurgitating not vomiting when they are bringing up food purposefully for young or mates) then your vet may suggest continuing meds or doing further testing.

If you are already noticing a better behavior then it does sound like your baby is on the mend. Good Luck and let us know how it goes!