You're off to a good start with nutrition (vitamins)!
I would recommend giving yourself scalp massages and brushing in circles at your scalp.
Also, mint shampoo stimulates hair follicles and helps them to grow faster.
I use Aveda Rosemary Mint, I have also used Organix Peppermint shampoo.
As for your hair being fragile, go easy on the heat and styling tools. When you need to use them be sure to use a heat protectant
Those make hair fragile. If you bleach your hair STOP, that always makes hair fragile.
If you already don't do those, try using different damage products.
I like Aveda's Damage Control spray. Use it on wet or dry hair. It also works as a detangler.
Some people have naturally more fragile hair, which unfortunately you can't do much about. Be sure to take good care of it!