What is this devilry?
Most cats are afraid of the vacuum cleaner, and this is completely normal. It's a huge noisy thing that, according to everything a cat's instincts are telling them, could very well eat them in one bite.
There are some felines that simply avoid it, but if Mittens is so afraid that it interferes with their (and your) daily life, then it's time to take action.
Here are some tips to improve their relationship with the nefarious vacuum.