If one decided and said that one will not buy a certain vitamin and then after further research decided to buy a vitamin does one need to matir neder.
b) Very similar question, if one says I will not give tzedeka over the phone but one time makes an exception and gives tzedeka to an important cause over the phone is this a problem of breaking a neder?
The question of nedarim applies only to decisions that relate to performing a mitzvah: If I begin to perform a certain mitzvah or positive custom, and then I wish to stop doing it, I need to be matir the neder.
However, for ordinary matters that do not involve the performance of a specific mitzvah, there is no need for hataras nedarim, and it is permitted to change one’s mind.
We change our mind about many things the whole time, and this is a legitimate part of human nature. When we wish to stop doing a mitzvah or positive deed, however, which we began to practice but now find difficult (say, wearing wool tzitzis, which are preferable to cotton, or being particular to eat specifically bread for melaveh le-malka), hataras nedarim is required.
Therefore, for the issues of vitamins or for a practice of not giving tzedakah over the phone – both ideas that don’t involve a mitzvah practice – there is no need for hataras nedarim.