1st off you need to see how often the compressor is cycling. If the compressor goes on and off every 3 to 5 seconds, or so, you most likely have a leak and are out of refrigerent.
At this point you can't simply vacuum the system down. This will just introduce more moisture through the leak into an already contaminated system. I use pressurized nitrogen gas, which is very clean and has no moisture to find leaks. Once the leak has been found and fixed the vacuum is then used to remove the moisture from the system. I will sometimes leave a vacuum pump on a car for a long time to ensure all the moisture is removed. The insides of the ac do not do well with moisture, you should get this fixed soon providing you truly have a leak.
P.S. Don't run the defroster until an uncharged system is fixed, or you have a very good chance of toasting the compressor altogether.