A 38-year-old woman from rural Maharashtra presents with a 6-month history of progressive weakness, numbness in her feet, and difficulty walking. On examination, she has loss of vibration sense and proprioception in the lower limbs, brisk reflexes, and a positive Romberg sign. Her diet consists mainly of polished rice and dal with minimal vegetables or fortified foods. Hemoglobin is 9.2 g/dL. Which vitamin deficiency is most likely responsible for her neurological findings?