1. Split vanilla bean lengthwise with a paring knife and scrape seeds from each half; reserve pod for another project. Combine water, corn syrup, sugar, butter, and vanilla seeds in a 3-quart, stainless steel saucier set over medium-low heat and gently stir with a fork until clear syrup is bubbling hard around the edges, about 5 minutes. Increase heat to medium-high, and cook, without stirring, until the syrup is golden, about 10 minutes.
2. Meanwhile, lightly grease an aluminum baking sheet. Measure baking soda and salt into a ramekin.
3. When the candy is ready, turn off heat and stir in baking soda and salt with a heat-resistant spatula. Stir in peanuts, then scrape onto prepared baking sheet and spread into a roughly 12-by-8-inch layer. Cool to room temperature, about 1 hour.
4. Chop or break brittle into 2-inch pieces. In an airtight container, peanut brittle will keep for about a month at room temperature, or up to 6 months in the freezer.