With car AC, it's run off of a drive shaft. The most common occurance is the gasket dries out during the winter causing the escape of gas. There's not necesarily a proper leak. Run the AC once a week even in the winter would prevent this.
If we're going to do it properly, the system would also require a charge of oxygen free nitrogen and purged from the opposite side as the charge was added to remove the moisture. None of this vac and charge bollox.
If the system is still holding some of its charge, and has not gone into negative pressure it wouldn't have pulled air (moisture) in and would not require OFN or vaccing out. Fgas also applies to systems with a charge of >3Kgs, which car systems generally don't have.
If it's just a recharge, phone your local refrigeration engineer, he'll be cheaper and have a better knowledge than a mechanic of the system.