I think the thing to remember here is static discharge. Todays MainBoards are coated so moisture isnt as big an issue as it used to be. BUT a MB has power applied to it at all times. This would be your cmos battery. So even if you disconnect your power supply and unplug all the cables your MB is still powered and Small shorts in the cmos system can render your machine unbootable.

Now take a vacuum hose with air flowing through it creating a huge static potential. I would estimate around 10,000 - 200,000 volts. more then enough to generate a little spark between it and your finger. now move this near a powered chip on your mainboard and POW.... BU bye.

The only proper way to clean a system is with residue free canned air.

Now if you insist on using a vacuum allthough why you would risk it I don't know I would recommend getting some nice bare copper wire and wrapping it around the end of the vacuum hose and attach one end to the case on your pc. Disconnect the P/S from the mainboard but leave the P/S plugged in(this will ground your case.). now hold the case with one hand and vacuum with the other taking care not to directly touch anything in the system. (I would unplug the P/S before cleaning the P/S itself as there is a risk of electrical discharge).

__________________
Just because something is improbable Doesnt mean its impossible.