You can feed vapor, however, at any point in time the compressor will be seeing the wrong composition gas. At first the vapor will be rich in the higher pressure, higher capacity component. This will cause high discharge pressure and temperature, high motor amps, etc. As the cylinder empties, the compressor will see the lower capacity gas which is left behind, changing the operating conditions the other way.
It will take some time for the "locally fractionated" gas to get mixed back into the original composition. Besides, if you need to charge the whole bottle, it's faster to put it in as a liquid.