Slice the beans open lengthwise and scrape out half the paste. Use the paste for another recipe, such as vanilla ice cream.
Place the spent vanilla beans (with half the paste still in the beans) in a clean jar and cover with the vodka.
Put the jar in a cabinet for a month.
After a month, open the jar and see if it smells like vanilla or vodka. If it still smells like vodka, put it away for a few more weeks and try again. When you get that nice vanilla extract aroma, you can start using it.
Top off periodically with more vodka. This should last for many months. When it stops smelling like vanilla, replace the beans or add another one.