How to groom a cat who hates the vacuum
Almost every failed grooming session fails the same way. The device switches on, the cat leaves, and the person is left holding a brush in an empty hallway wondering what they did wrong.
They usually did one thing wrong, and it happened before the brush ever touched fur.
The problem is sound, not touch
Cats hear roughly an octave and a half above us. A grooming vacuum running at 82 dB is, to your cat, closer to what a running dishwasher sounds like pressed against your ear. Most cats do not object to being brushed. They object to being brushed by something that sounds like that.
This is why the first decision that matters is not technique. It is the machine. Below about 70 dB most cats will stay in the room long enough to find out what is happening. Above about 75 dB, most will not, and no amount of patience fixes it, because you are asking the animal to override an instinct rather than a preference.
Let the machine be furniture first
Put the unit on the floor, at the lowest setting, somewhere your cat can see it. Then do nothing for a full minute. Do not approach, do not call them over, do not hold out the brush.
What you are doing is letting them file it under furniture rather than under threat. Cats categorize fast and then stop paying attention, which is exactly what you want. A minute feels absurdly long while you are standing there. It is the highest-yield minute in the entire process.
If your cat leaves during that minute, let them. Try again tomorrow. You have lost nothing.
Start at the shoulders
Not the tail. Definitely not the stomach.
The shoulders and the upper back are neutral ground on almost every cat. They are also where cats groom each other, so the sensation already means something friendly. Work backwards from there in short passes, following the direction the coat grows.
The stomach, the legs and the base of the tail are negotiations for later, and on many cats they never become available at all. That is fine. Most of the fur that ends up on your sofa comes off the back and flanks anyway.
Five minutes is a grooming. Twenty is a grievance.
The instinct, once your cat finally settles, is to keep going while you are winning. Do not.
End the session while the cat is still content, even if you are only half done. A cat who walks away calm comes back tomorrow. A cat who was held for one minute too long remembers that minute, not the nine good ones before it, and the next session starts from further back than the last one did.
Two short sessions a week beat one long session every time, and they beat it by a wide margin.
Never force a knot
If the brush stops moving, you stop moving.
Pulling through a mat hurts, and worse, it teaches your cat that this activity sometimes hurts without warning. That is how you lose the trust you spent three weeks building. Clear the brush head, or switch to a clipper and take the mat off at the base.
Anything already matted is a clipper job, not a brush job. A brush works on loose undercoat. It does not undo a knot, and trying is how people end up at the vet with a cat that will not be touched at all.
If your cat has already decided
Some cats will not accept a motor. Not quietly, not at any decibel level, not after any amount of patient introduction. If that is your cat, you are not doing it wrong. You have a cat with an opinion.
For those, a misting brush is the answer instead of a vacuum. There is no sound at all, and the light mist means the coat is damp while you work, so knots give way rather than pulling and the loose hair stays on the teeth instead of drifting across the room. It is slower and you do the collecting yourself. But it happens, which beats the vacuum that stays in the cupboard.
What to expect
Most cats settle within the first two or three short sessions. A few take a couple of weeks. A small number never sit for a motor and always will.
The thing to hold onto is that you are not training the cat to enjoy grooming. You are building a routine boring enough that they stop having a view about it. Boring is the goal. Boring is what a cat will tolerate indefinitely.