Do NOT use a vacuum to clean dust from your PC. Not inside, and not outside. Vacuums create static discharge, and that is a BAD thing for computer hardware.
ONLY use compressed air from a can or from an air compressor, in short bursts of 3-4 seconds, to avoid frosting components, when cleaning the inside of your PC. You CAN technically probably use a brush with long, soft bristles, but you'd still need to use air to actually get the dust OUT of the case, so it's kind of pointless. Plus, there is likely an argument to be made for that causing ESD or static discharge as well. I'd recommend only using compressed air from either a can or an air compressor.
Yes, that is fine. And actually, you can probably wipe down SOME surfaces inside, like the inside of case panels, the top of drives, controller boxes, fan blades, etc., IF you are extremely careful. Be sure to power off the system AND flip the switch on the back of the PSU to the off position before doing ANY kind of work inside the case.