My surgeon had me start the vitamins as soon as I could tolerate them... the multis made me nauseous for a couple days, but everything else was fine.
As far as the hair loss, it's called telogen effluvium and is triggered by any trauma to your body. That includes both the significant surgery we have, as well as the greatly reduced calories, but may also be (for some people) things like car accidents and child birth. There is nothing other than not having the trauma that will prevent it, although people respond differently (for example, I had no hair loss after a nasty childbirth, but my SIL lost a lot after a mild childbirth. It's really variable).
Your body is cycling through your regular hair loss/regrowth cycle faster than usual, which means you lose your longer hair and grow new hair. As a result, you look shaggy and unkempt for a while. That's why many of us cut our hair shorter during this time as it masks some of the shagginess.
As far as regrowth, my reconstructive guys told me to keep my protein at about 100 grams per day (yes, that's higher than most recommend. Your mileage may vary), keep up with my iron, C and zinc 2 or 3 times per week.
I had loss, but the regrowth came in fine, and it wasn't really noticeable with shorter hair.
Circumferential LBL, anchor TT, BL/BR, brachioplasty 12-16-10 Drs. Howard and Gutowski
Thigh lift 3-24-11, Drs. Howard and Gutowski again!
Height 5' 5". Start point 254. DH's goal: 154. My guess: 144. Insurance goal: 134. Currently bouncing around 130-135.