Place almonds and vanilla pod in a bowl; add enough water to cover completely. Place dates in a separate bowl and add enough water to cover completely. Soak almonds and dates at room temperature, 8 hours to overnight.
Drain and discard soaking water from almonds and rinse almonds thoroughly. Drain water from dates and keep water to add to the almond milk, if desired.
Combine soaked almonds, vanilla pod, dates and sea salt in a blender; pour in 750ml cold water. Blend on high for 1 minute. Let sit to allow foam to subside, about 1 minute. Add remaining 250ml water and some of the date water for extra sweetness, if you like; blend until almond milk is smooth, about 1 minute.
Place a nylon or muslin strainer bag in a litre-size measuring jug or large bowl. Pour almond milk into the bag and lift to drain. Gently squeeze the bag to extract as much milk as possible. Pour into clean sterilised bottles and store in the fridge.