Vodka at the very minimum. I've had this conversation with two pro brewers (including one from Sam Adams). Raw vanilla beans are absolutely loaded with bacteria. The SA guy said it's the most troublesome ingredient they've ever used. Both pros pasteurize their beans in sealed containers, and then flush the whole thing into a bright tank without ever exposing the beans to air. Keeping it sealed this way they manage to keep most of the aroma, and still avoid infection.