They are talking about vitamin supplements, not literal vitamins that you need in order to live. Vitamin supplements do not survive in a budget reduction spreadsheet - they're easy to let go of for a while. On the other hand, if you're in pain, you need painkillers, and you're not going to be thinking about your budget, you're just going to go get some to get rid of the pain, even if it's just a temporary fix (and even better for the business if it's just a temporary fix - recurring revenue!).
That's the whole thing, the whole "solve a real problem" thing they keep talking about for startups.
I think supplements would have been a more effective word choice for the analogy for me, as vitamins reference something that is required for life.
Eat a balanced diet, go outside sometimes, and you'll be fine.
One single cherry tomato contains enough vitamin C to last you for a week. If you take a vitamin C tablet too, you're just making your pee more expensive.
Vitamin C is presumably not saving any meaningful number of people from scurvy in the developed world, but it does seem to be at least a darn good placebo. There's more to health than avoiding deficiency diseases; and we're talking about typically pennies per day here anyway. A small amount compared to the food it's supplementing.