The best way to help a dog become less scared of the vacuum cleaner is to start by desensitizing them to the sound and sight of it. Start by leaving the vacuum in the room with the dog, but turned off. This will allow them to become familiar with the sight of it. Then, turn the vacuum on for a few seconds, then turn it off and reward the dog for remaining calm. Gradually increase the amount of time the vacuum is on and reward the dog for remaining calm. Eventually, you can start using the vacuum cleaner around the dog while rewarding them for remaining calm.