The recommendation is not to vacuum computers because the vacuum hose can develop a very strong static charge. If you have access to a compressor that's good, otherwise lung power can get many of the dustbeasties out.
I was once vacuuming out a bunch of old antiquated terminal server gear which had a whole bunch of dust clumps all over the inside of the chassis. At the time I didn't think anything of it, and used the house vacuum to clear out the machine. Of course the vacuum hose wasn't grounded and happened to be made out of plastic, so the static charge that was built up by the dust moving down the hose was rather impressive- enough to leap a 2" air gap with a sharp bang straight into the PCB.
That piece of equipment never operated properly again.
If you want to vacuum computer equipment out, please please please invest in a proper ESD safe vacuum cleaner. They cost anywhere from $100 to $300, the most typical models (which were sold by 3M and about a dozen other companies) resemble a black toolbox with a handle. The hose on those machines is electrically grounded via a three prong power plug, and the entire assembly is rated to suck up something as fine as toner without igniting it.
If you don't want to do that, then buy a $5 can of compressed air and use that instead.
Please, please please don't use your house vacuum cleaner. You can permanently damage your equipment, even if you're being careful with it.