Cut the 10 vanilla pods in half lengthwise. Coarsely grind 4 of the pods. Combine in a sealable bottle: the ground vanilla, the remaining 6 whole vanilla pod halves, 750 ml grain schnapps, 150 g vanilla sugar, and 250 g brown sugar. Seal the bottle and shake well. Store in a cool, dark place for 6 weeks, shaking daily to help dissolve the brown sugar.
After 6 weeks, strain the liqueur through fine cheesecloth or a fine sieve. Keep the 6 whole vanilla pod pieces. Discard the ground vanilla and other solids. Pour the strained liqueur back into a clean, sealable bottle and add the 6 vanilla pod pieces back. Seal and let stand in a cool place for 2 weeks to mature.
The vanilla liqueur is ready to use. Serve in baking, desserts, or as a digestif. Store in a cool, dark place where it will keep for up to 1 year.