Gaining

I was discussing gaining methods with someone, they talked about using vitamins (C and b complex) for a week then cutting them off
Supposedly that withdrawal made them gain weight
Anyone has any experience or knowledge regarding that to back it up?

Don't do this. Your body needs both to function. Being malnurished leads you to getting sick. And if your normal diet gives you enough vitamin C and B complex, taking supplements is a waste of time and money.

My hormones are messed up from gaining so I would suggest you do not ignore the importance of nutrition! Get supplements IF you are not getting enough, like Vit D. Reversing the damage is a pain.

Here here

I have seen a number of gainers and feedees play fast and loose with their health in the name of getting fat. Never ends well.

Your body takes note of whatever damage you do to yourself and repays you with interest. Anyone over the age of 30 can vouch for this.

When I regained weight, I put on over 400 pounds in 3.5 years through stuffing multiple times a week, eating 10 to 25,000 calories a day even when not stuffing, and a steady diet of bread, cheese, pasta, pizza, potatoes, and fast food. I was just over 600 lb when I had going to the hospital with a cardiac emergency, but I had vitamin deficits of vitamin B, C, potassium, and magnesium. It's definitely very possible to have a massive calories surplus and a vitamin deficit.

I get into a shocking number of arguements with people in and out of the community about malnutrition in obese people. They conflate being fat with starving.

If you got fat while not eating a balanced diet or not taking a multi-vitamin, the missing nutrients are not going to magically spawn into you. You still have to comsume them.