Matilda used to be petrified of the vacum, same thing running & crying and hiding. We used to leave it out with the cord out in the hallway for a few days (this is the same way I desensitise dogs so I figured I'd try it out) and then while she & DH would have a shower or something else, playing like dachlostar said, I would vacum in another room. For a short time & then turn it off & put it back in the hallway. DH & I would ignore the vacum while it was in the hallway and before bed we would say "time to put the vacum to bed" and she would always help us.

We just did that heaps for a few months & now she will watch me closely when I get it out and get overly helpful and insist on vacuming herself.... ah well...