If your system is only a little low, just use the cans.If you get one with a little gaugeface on it, it will show you pressure,read the directions on the can and you wont overcharge it.You only need to vacume the system down if you got air in the system.Its not a big deal.Non-condensables (air) will only get in the system if it is out of freon, or very low with the compressor pulling a vacuume.

(I am a nate certified commercial HVAC tech)