Back in the good old days when I still had lots of hair I had a hair dryer that would operate just fine on a 15 Amp circuit. People with even more hair wanted bigger hair dryers ( a blow torch with a blower ).
In my opinion a 20 Amp. circuit started to be required for a bath room receptacle to accomodated one appliance, the hair dryer. I don't remember anyone thinking that this was the dumbest idea ever. There are curling irons and hair dryers that will work on a 15 Amp circuit so why was a 20 Amp circuit required?
You don't hear anyone complaining nor trying to get the requirement for a 20 Amp. bathroom circuit changed. Shouldn't you be allowed to run two 15 Amp circuits?
When you say you may need a 20 Amp circuit for a vacuum cleaner just what makes that so different from needing one for a hair dryer.