Vacuum cleaner belts are not repaired, but are replaced.
In the gas tank
Yes you may its not going to hurt it. Also while you do fuel system cleaner use injection cleaner too.
It was discontinued and replaced with Soilmax
Add a fuel injection cleaner into the fuel tank or go to a garage that has a fuel injection service machine.
The answer is oil. Injection cleaner will help on a car with higher mileage.
Pouring the injector cleaner down the throttle body won't clean the injectors. You need to add the injector cleaner to the fuel tank for it to help you. Read the instructions on the container of injector cleaner.
You can tell that your vacuum cleaner needs to be replaced when it quits working properly. Signs of that include the belt quits turning or the vacuum quits picking up dirt.
Modern fuels have a fuel injector injector cleaner built into them. However I do recommend you use a good fuel injector cleaner such as Seafoam or Chevron Techron once a year.
If you use economy gasoline, add cleaner every time you change the oil. If you use top tier fuels (from major companies), then never add cleaner. vbd