1. Low lying placenta at 20 week scan:
Placenta generally migrate as gestational age advances. So please repeat ur scan by 28 weeks to know Babys growth, Liquor level, Doppler and Placental localization.
By any chance if it is still low lying then avoid too much walking or exercise.
2.
Vitamin D deficiency: U took proper Vitamin D supplements so nothing to worry.
3. Continue Iron and
Calcium supplements. Add protein powder for supplements also.
4. Diet in pregnancy: U can buzz me for detailed chart. Have high protein diet.
Atleast 6 diet shud be there. 3 major, 3 minor diet. Including lots of salads, daily green leafy vegetables, add Sprouts atleast 2-3 eggs (3 egg white and 1 yellow), U can eat boil chicken twice a week, Daily approx 1 Litre milk and milk products like Paneer, Curd, Butter milk, Yogurt.
5. Normal Vaginal Delivery:
Many factors affect the mode of delivery. Like baby's position, Liquor level, Doppler, Baby's growth and Ur progress during labour process and baby's has passed motion during labour or not. So it is too early to comment on whether u will have Vaginal delivery or not.
I would sugget rather than stress about ur mode of delivery, U enjoy ur process of pregnancy. Do daily meditation. Eat healthy diet. Talk to ur baby daily, get emotionally attached with the baby. And just enjoy the new phase.