Can't gain weight by eating? Have you got norovirus or something? Check that food is going in the right orafice otherwise it'll have to be inverse liposuction.
For a multi-vitamin shake I'd get a milkshake from the supermarket and use it to down a vitamin tablet.
I wasn't being 'literal', yes I can put weight on by eating more but I have to eat a ridiculous amount of food and even then I barely put anything on and it comes off very easily, I know nothing about this subject so maybe it's a fast metabolism or something ?, when I was younger people as always astounded by the amount of food I could eat without putting it on, many people said when I reached my 30's things would change but that hasn't been the case yet.
Anyway like I said in my other post I have been using slim fast shakes with full fat milk and bananas with my normal diet which usually consists of,
Breakfast : Orange juice, a large boil of cereal with sliced banana.
Lunch : Pasta salad or pasty/sandwich, crisps, cake & a soft drink.
Dinner : A large mixed vegetable or Spinach curry, white rice + nan bread (I have this 5 days a week)
Tea : A average sized curry, usually dhal (lentil) curry (no oil), and white rice.
I usually try to have at least two pints of ice water through the course of the day although I normally have more tbh.
And then when I get home I normally have a couple of pints of lager and sometimes snack on some jaffa cakes or something similar.
I'm not sure if that is considered a lot, I'm not sure how much I weigh but people say that I'm quite thin, although the shakes seem to of helped me put on a bit of weight recently, about a stone, probably the full fat milk has helped more than the anything tbh.