You can't use your vacuum's dust filter for your fans intake. It requires a different static pressure and would most likely, depending on the filter, reduce airflow drastically.
You might be able to get away with using your vacuum's blower to clean your PC. Just accept the fact that you won't get every little bit out and make sure your PC is grounded. This of course assumes that your vacuum blower dosn't exhaust dust and it has sufficient power.