I like aged white rum (Havana Club 3 year old, if you don't want to pay absurd amounts). Something that's had the rough alcohol taste rounded off a little, but also not too much of barrel / other flavours that'll overwhelm the vanilla either.

Split the beans in half, stuff them into the bottle, put it in the pantry. Shake it once a week. Should be good to use in about 2-3 months.