If one recites a blessing on a glass of grape juice, drinks some of it, and then pours the rest of the grape juice back into its original bottle. He would like to drink grape juice in a different meal, can he recite the blessing "Bore Peri HaGefen" on the grape juice in the bottle that had been mixed or would it be considered a Brachah Levatalah (Blessing in vain)?
"Bore Peri HaGefen" is Birkat Hanehenin (blessings over benefits). It is a person's duty to thank His Creator for the pleasure he benefits from this world. The blessing that a person recites on food allows him to eat only the things he had in mind while reciting the blessing and is limited in time until he is distracted from the blessing. From a Halachic point of view, the blessing has no direct effect on the food. In the case you mentioned we are talking about the blessing of "Bore Peri HaGefen" after a distraction as it is a different meal, therefore one must recite "Bore Peri HaGefen" not only in the case of a bottle of grape juice mixed with grape juice which he blessed already, but even if it was in the glass itself without any mixture. (Shulchan Aruch Orach Chaim 179) In regard to Kiddush on wine which was drunk from, please click here see Rabbi Yoel Liberman's responsa , and Shulchan Aruch 182 & 271. See also Shemirat Shabbat Kehilchata 47, 15.