I'm growing my hair really long just now, I used to have terrible hair it wouldn't grow, it was permanently between my chin and shoulders.
I started to put coconut oil onto my hair every couple of days, in the evening, on dry hair just completely saturate it and leave on for a few hours or overnight, when you wash it out, you need to put shampoo straight on, without wetting hair first, then wash as normal. I do this less often now that my hair is in good condition, maybe once per fortnight. I also used to blow dry and/or straighten my hair everyday, so I've stopped that and on days I've not washed my hair I put it in a bun or tie it up in some way so it still looks ok.
I never go to the hairdressers, I had a bad experience about 9/10years ago, I bought a pair of hairdressing scissors from boots and just snip off any split ends that I spot, rather than trimming the whole of my hair. I use a tangle teezer now and never any other kind of brush or comb, I know some peoples don't like them, but it works for me.
For vitamins/ supplements I think unless you are actually deficient in anything then they will make little difference, in my experience. Maybe you are deficient in something, you could get checked?
My hair has gone from around an inch below my chin to just past bra strap length in about 2years, I never thought that was possible for me! It can definitely be done, just need to get the health back. I know it's a bit of a Pinterest cliche, but I really recommend using coconut oil or also burdock oil, (as in dandelion and..) I get it when I visit home, but I think it's on Amazon as well.
Good luck 