Chop 2 vanilla beans, place in a blender and pulse until coarse. Slit the other beans in half lengthways and add both to the glass jar.
Pour the vodka over the beans, making sure they are completely submerged (depending on the dimensions of your jar, you might have to chop them up to fit).
Cover the jar and shake well. Store in a dry place for at least one month to infuse - the longer the better. Make sure to shake the bottle every week until ready to use. Taste the extract and let it sit for longer if you want it more potent.
After you are happy with the strength of flavour, you can strain if you prefer and decant into another bottle to store (I skip this step and leave the beans in).
Notes
Adapted from TheKitchn
Recipe by The Blurry Lime at https://www.theblurrylime.com/diy/homemade-vanilla-extract/