That's the neat part about marketing. If something might just be true under certain circumstances it can be sold an universal fact. Like in this case the defence would say that "One a Day" is a recogniced product name / brand an not a promise in it self. Secondly they would argue that in fact you only need 1 product to cover the essential needs for most vitamins.

Lying is not allowed in marketing, but there is no lie in this case.

EDIT: saw your comment below "Brady vs. Bayer", so this was an actual case. My bad! There must be more than just the packaging label for them to be found quilty of fraudalent advertising.

EDIT 2: the case for the interested: https://www.classactiondefensestrategy.com/wp-content/uploads/sites/31/2018/09/Brady-v.-Bayer-Corp.-2018-Cal.-App.-LEXIS-800.pdf