Preheat the oven to 180 degree ( 350 F)
Mix the flour with the sugar, vanilla and baking powder
Add the oil, mineral water and apple cider vinegar and mix until well combined
Wash the rhubarb and cut it into pieces (ca. 1 cm)
Add to the dough, but keep some pieces for topping
Line a muffin tin with paper and add the dough, top with the remaining rhubarb pieces
For the streusel: mix all ingredients (butter in pieces) and kneed into a crumbling dough
Crumble on the muffins
Bake for 30 minutes