A good vanilla pod arrives supple. It bends without cracking, it smells like more than sweetness, and it splits cleanly under a knife. Storage decides how long it stays that way.
The short version
Keep beans in an airtight container at room temperature, away from light. That is the whole method. Everything below is the reasoning.
Use glass, keep it closed
Once a sealed bag is open, move the beans to a glass jar with a tight lid. Glass holds no odors and lets you check the pods without opening anything. Keep the jar in a cupboard or a drawer. Light and heat both age a pod faster than time does.
Skip the refrigerator
Cold storage sounds careful and works against you twice. The dry air of a refrigerator pulls moisture out of the pod, and every trip out of the cold invites condensation onto its surface. Damp plus a sealed jar is how mold starts. Room temperature is the safer setting.
Air the jar now and then
Every few weeks, open the jar for a few minutes and let the beans breathe. This releases any humidity the pods have given off and lets you check them by eye and by nose. It takes less time than making coffee.
Frost or mold? Look closely
Sometimes a well kept pod develops fine white crystals. Look closely before you worry. Vanillin frost sparkles, sits in needle-like crystals, and smells strongly of vanilla. It is a sign of a rich pod, not a spoiled one. Mold is different: dull, fuzzy, usually in patches, with a damp or off smell. Frost you keep. Mold you do not.
If a pod dries out anyway
A stiff pod is not a lost pod. It still carries flavor, just less water. Split it for a long infusion, drop it into a jar of sugar, or let it dry completely and grind it. The flavor is still in there.
Stored this way, whole beans keep their character for many months. Buy what you will use, keep it closed and dark, and the last pod in the jar will behave like the first.