I am leaving for a vacation and leaving my rMBP. How should I store it except from shutting it down? How much charge should I leave in order not to lose too much capacity?
You are overthinking this as there is nothing special about the lithium battery that requires special storage considerations. Just store it like you normally would; I put mine on a bookshelf to hide it from the inevitable human curiosity of the dog walker/plant waterer when I go on extended trips.
Your lithium ion battery is going to lose capacity whether you use it or not because the biggest factor in the longevity of a battery is age not how much it's charged. Now, the amount that it loses over the month is going to be negligible; you won't even notice.
I posted an answer that is relevant to this question: MATH FINDS TOP CULPRIT AS LITHIUM-ION BATTERIES AGE
Just leave it plugged in over the month and you will be fine.
To quote directly from Apple about their own products and batteries: "Store it half-charged when you store it long term. If you want to store your device long term, two key factors will affect the overall health of your battery: the environmental temperature and the percentage of charge on the battery when it’s powered down for storage. Therefore, we recommend the following:
Do not fully charge or fully discharge your device’s battery — charge it to around 50 per cent. If you store a device when its battery is fully discharged, the battery could fall into a deep discharge state, which renders it incapable of holding a charge. Conversely, if you store it fully charged for an extended period of time, the battery may lose some capacity, leading to shorter battery life. Power down the device to avoid additional battery use. Place your device in a cool, moisture-free environment that’s less than 32° C (90° F). If you plan to store your device for longer than six months, charge it to 50 per cent every six months. Depending on how long you store your device, it may be in a low-battery state when you remove it from long-term storage. After it’s removed from storage, it may require 20 minutes of charging with the original adapter before you can use it."
See: http://www.apple.com/au/batteries/maximizing-performance/ as well as https://support.apple.com/en-au/HT204054 for further information on batteries in Apple products.