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An unopened bottle of vodka can be stored indefinitely. Because of this, you should wait to open a bottle until you're actually ready to start drinking it. The alcohol in the vodka starts to slowly evaporate once the seal is broken and the bottle is opened. It will take several decades before the opened vodka becomes undrinkable because the alcohol is a preservative. If your vodka is sealed with a cork, make sure the cork is firmly in place since a loose seal could speed up evaporation. If your bottle has a screw top like a cap, make sure it is tightly closed. If the cork or screw top doesn't seem like it will be secure, you may want to store your vodka in a different bottle or container. If you'd like to take vodka with you when you're out and about, store vodka in a flask. Most flasks come with small funnels so you can easily transfer the vodka to the flask. Just be sure to use the vodka or remove it within 3 days since prolonged exposure to the metal in the flask can make your vodka taste metallic. When you've emptied the vodka, rinse out your flask and let it air dry.
Keep the bottle sealed for as long as you can. Ensure the seal on your bottle is intact. Keep vodka in a flask for a very short term.