If it's still mounted inside the computer using a vacuum cleaner is a bad idea, it produces severe static buildup and should not be used inside a computer!
Canned air can get expensive and should only be used in lightly dusted situations for heavy dust build up it's a waste.
If you have an air compressor that's the best dust removal route even if it is running high air pressure, just keep your finger on the fan blade and don't allow it to spin fast, or you could damage the fan.
I have adopted a cleaning policy for every machine I work on, it goes first to the shop outside and I blow all the built up dust and crap out of the machine, before it's ever brought into my home, as you would be surprised at what I have found living inside some peoples nasty computers.
So to sum up:
Vacuum Cleaner should only be used on the outside of a computer case never inside.
Canned Air for lightly dusted situations.
Air Compressor for heavily dusted situations.
Good luck, and Best to you!
Ryan