Also, dustfilters in the case does help a bit, means you clean filters instead of components.
I might be entirely wrong here, but I'll blame my father for it; It should be fine to use a vacuum cleaner for cleaning your PC, just make sure that you somehow lower the air velocity. If you can lower the vacuums speed you lower the risk of discharges. I usually put it at the lowest setting and use a plastic bottle cap with a hole that I've threaded a 5mm plastic tube through.