Heat oven to 180C/160C fan/gas 4. Line the bases of 2 x 20cm sandwich tins with baking parchment and grease with a little of the dairy-free spread.
Put the dairy-free milk into a jug and add the vinegar, leave for a few minutes until it looks a little lumpy. Put half of the vanilla seeds and all the other cake ingredients into a large bowl then pour over the milk mixture. Using electric beaters or a wooden spoon, beat everything together until smooth.
Divide the mix between your two tins then bake in the centre of the oven for 30-35 mins or until a skewer inserted into the middle of the cakes comes out cleanly. Leave them in their tins until cool enough to handle then carefully turn out onto wire racks to cool completely.
While the cakes are cooling, make the filling. To make the vegan buttercream, whisk or beat together the dairy-free spread, icing sugar and remaining vanilla seeds until pale and fluffy. Dairy-free spreads do vary so if the spread you are using is quite soft try to avoid using electric beaters. Stir the ingredients together instead to avoid overworking it. However, if the mixture is too firm, use electric beaters to help lighten it and add 1-2 tbsp of dairy-free milk when whisking.
Spread the jam onto one of the cooled sponges, top with the buttercream then place the other sponge on top. Dust the assembled cake with a little icing sugar or caster sugar before slicing.