Charrin, would be a good idea to start your own thread, as your situation is a bit different from the OP's.
You say, " my baby was the one constantly screaming because he was hungry whereas everyone else's babies seem to be calm and sleep through the night."
The babies sleeping through the night would be a big problem - it is just not good for a newborn to sleep through the night. It is horrible when your baby is screaming, even so....especially if they fight at the breast when you are trying your best :( But don't assume the other mothers and babies were without their own difficulties.
The feed - express - feed cycle you got into so early is very pressurising, and it's not surprising this felt impossible to maintain. Yes, mixed feeding in your situation has to be better than stopping bf altogether - with 2-3 bottles at night from day 10 your breastmilk will (probably) gradually go, but who knows when, as experience is so variable, and you might continue for longer than someone else.
Now your baby is better at latching and the pressure of expressing is off, you might feel more able to go back to full bf - your midwife could help you with talking this through.
On the vitamins thing: the vitamins are really mainly for you, not for the breastmilk, and it is up to you if you think you need them. There is some evidence that in the UK where we have less sunlight, Vitamin D (one of the Vits in the supplements) is a good idea for bf mothers and this does go into the breastmilk. Most women who go outside every day with their babies and have a reasonable diet really don't benefit from these.
Vitamin D is added to formula as an extra. You could ask your HV if she thinks you need your own Vit D as well.
Hope things get better for you soon.