D.M.
I really like Linda C's idea to be out of the house when the vacuum is used since that would mean someone else would have to do the vacuuming for me! Seriously though, I used to turn the vacuum ON near my daughters' crib because it would make them fall asleep.
It's likely that when you stopped putting her in the backback and she was closer to the floor that she noticed that when the vacuum runs over something.... it disappears!
It IS possible she dreamed of it, as another suggested, or even saw a small toy or piece of string sucked into it and is at some level afraid that can happen to her too if she gets too close to it.
I do think it's a phase, and I think you're right in letting her touch it while you hold her. I think she'll be too heavy to hold while vacuuming when she's 16, but my guess is she'll be over it before then ::smiles::