Hey ... We are all happy that you would not use a vacuum ... and ... A rock can also cause static electricity ... prolly more than a vacuum (I am certified 3 different ways.)
Nobody can make you use a vacuum ... or make you eat broccoli ...
Your vac is no more threat to your PC than you are.
1) Aleays leave your PC PLUGGED IN to a properly grounded (three prong) electrical outlet. When you are asleep ... When it is on ... When you are swapping cables ... When it is off ... When you are playing games ... When you are on vacation ... AND ... *Especially* when the cover is off and you are poking around, in there. (working, cleaning, swapping, anything. So long as an active toddler is not playing in the room, or you are not moving the PC to another outlet, LEAVE IT PLUGGED IN !
Whenever the computer is plugged into a grounded outlet, the PC is GROUNDED.
2) WHENEVER YOU APPROACH THE PC ... ALWAYS TOUCH BARE METAL ON tHE CHASSIS ! Whenever you are going to touch the System Case, Reach over and touch the top corner of the back of the case (the "Chassis, Proper").
CONGRATULATIONS!, You have just equalized any charge differential between your body and "chassis ground". EFF EMM ("No Static At All"). Steely Dan.
3) Do not hold onto a "Van de Graaff" generator or a Tesla Coil while vacuuming your PC !
http://www.boston.com/ae/theater_arts/exhibitionist/generator.jpg
http://www.salrestivo.org/NikolaTesla.jpg
4) Now ... If your cargo shuttle has crash-landed on a frozen desert planet, where the humidity is less than 30%, and you are stuck in a cave (with your PC, coils and generators). and if your Jedi Frock is woven from the fine hairs of some exotic, alien goat species ... And, if you have been using the plastic wand extention of your cave-vac to practice your light-sabre skills and rubbing the wand all over your fuzzy frock, while also vacuuming the innards of your open PC (that is not plugged in), ... Well ... then ... You might have a static problem !
Today, (true) the largest solar flare ever recorded occurred ... and, also, a BIG lightning storm passed right over my head (also quite true) ... Now, I KNOW I SHOULD have unplugged my computer and my TV, when that storm blew over ... but I did not ... How could I watch the Weather RADAR ??
My TV and my PC both survived the day !
Nothing against the little green guy, tho ... really ... just that he seems a worry wart and, with a bare modicum of care and common sense, it is PERFECTLY SAFE to use a vacuum on your PC ... BUT ... If the hair on your little Jedi chest starts to stand up .... RUN !!!