To make it crispy try to make the sabudana mixture dry. 1) Do not over-boil the potatoes. If potatoes are overcooked then they will add moisture to the sabudana mixture. 2) The sabudana pearls need to be soaked perfectly. If it is too soft or soggy (it happens when you add more water to soak the sabudana) then it will add moisture to the sabudana mixture. 3) Fry the sago vada in medium hot oil. The oil for frying the vada should not be too hot or too cold. 4) Fry them over medium heat, flipping once or twice, so that sabudana is crispy outside & cooked well from inside.