Gastric Bypass Cost

I realize that people who undergo gastric bypass surgery need more vitamins like B-12, possibly Iron, B-1 or even Calcium, but when choosing a vitamin, does the form need to be taken into consideration, since absorption is different in a bariatric patient?

VLCD (very low calorie diet) people eat like 500-800 calories a day and so do gastric bypass post-op patients. So, I weigh 180 pounds and if I start eating 600 calories a day to lose weight, wouldn’t it be the same as if I went in and got a gastric bypass? What is the difference medically if someone would just go on a diet similar to having the bypass done?