Keep in mind that even if the medication kills all of the bugs, the dandruff particles will still be stuck in the fur. Try brushing as much of the gunk out of your bun's fur as they'll allow and see if it comes back, but be gentle on the bunny skin (and do it outside in case the bugs are still alive!). If you have access to something that magnifies (magnifier glass, loupe, microscope, or really good eyes), you could look for bugs yourself--cheaper than going back to the vet.
I'm interested in what medication that was too. I just went through this with my house herd and used injectable Ivermectin, but even that has to be followed up with multiple shots and a thorough cleaning of the cage/litterbox/accessible areas until the mites all die of poisoning or age. Mites suck!
