At 5, she is still trying to figure out the difference between the truth and pretend. That is why her "lies" are not to get her out of trouble. She is learning about truth and what she would like it to be. (That is why children this age get frightened of ghosts.) In the situation with the vitamin, my response would be something like "so you wondered what would happen if you said you didn't get it?" I would not label it a lie. I would label it a fantasy or make believe. She will gradually learn the difference by your labeling it as make believe and talking about truth.
My granddaughter, during kindergarten, told me stories such as her aunt living across the street from school. She insisted it was true even after I said she lived in Vancouver. Then, I said that it sure would be fun if she lived across from the school. My granddaughter beamed and said that was true. She never said her aunt lived across the street again.
I know a mother who kept insisting that her daughter was lying to her at that age even tho, what she said made no sense as a lie. Iit was pretending. She labeled the daughter as someone who lies and the daughter does now lie as a 12 yo. Kids do live up to our expectations.
I urge you to go easy on labeling what she says as a lie. Help her to understand the difference between what she thinks or wants it to be and what is actual.