While this seems theoretically possible, I would not advise you to use this method to address moldy vents. When professionals clean vents out they use a tool called a bladder to isolate spores. If you were to use the fogger in your a/c return and turn on the fan, you may release spores that could eventually make their way into habited zones. There is also the matter of the mold and mildew disinfector and inhibitor to aim attention at. Ducts need to be properly cleaned and moisture removed before the actual fogging. Although I am a big fan of do-it-yourself home repair, this is one of those jobs you really need to get a professional for.