How long does pure vanilla last?

Assuming you mean liquid vanilla extract, it will stay good forever. The vanilla essence was extracted using alcohol, and the finished product is typically 35%alcohol, so it's not a bacteria-friendly environment.

The reason most of us keep extracts is that fresh vanilla beans, like any produce, will grow moldy and rotten if not kept in an airtight container. Even airtight, the beans will eventually dry out and not be good for much but scenting your sugar canister, so extract is your best long-term bet.