Prepare the dough. Place water, salt, sugar, and oil in a large mixing bowl and stir until sugar has dissolved. Add the flour and stir to combine using a spatula or rolling pin. Knead into a cohesive ball. Cover with a tight lid and rest for 10 minutes. Uncover and knead again until a smooth top forms. Divide into 4 equal pieces and form into balls.
Add a tablespoon of oil into a plate or 8" cake pan. Coat each ball generously with oil. Cover and rest at least 1-2 hours or overnight in the fridge. For the overnight dough, thaw to room temperature before use.
To stretch the dough, grease your palm and the work surface. Take a dough and flatten it into a thin circle using the heel of your palm. Then, flip and twirl or stretch to thin out the dough as much as you can, about 4-5 times larger than the original size.
Top with sliced banana, vegan butter pieces, and sprinkle with sugar. Bring the sides to the middle and overlap the edges slightly to seal. Repeat with the top and bottom edges to get an enclosed large rectangle roti.
Cook the roti after you fold it. Heat a pan with vegan butter or oil over medium heat. Place the roti in the hot pan, seamed side down, and cook until golden brown, about 3-4 minutes. If it browns too quickly, lower the heat. Flip and cook for another 3-4 minutes, adding more oil if needed. The roti will start to puff up during cooking and you will see the banana starts to caramelize. Repeat the same process with the remaining dough.
Serve warm with a side of dal or as-is.