Concerning grape juice (e.g., Kedem, Nature’s Own, etc…) that is used for Kiddush on Leyl Shabbos and Leyl Yom Tov (& consequently during the daytime meals). If after making Kiddush, I pour an amount (1/3) of grape juice back to the original bottle and add water to my cup (to dilute), would I still fulfill the requirements of Kiddush?
After making the Borei Peri HaGafen blessing, you have to drink wine or grape juice.
Some authorities are stringent with regard to grape juice (unlike wine), and maintain that if grape juice is diluted with water it loses the Borei Peri HaGafen blessing. Although other authorities are lenient concerning adding a small amount, this will only be true insofar as the grape juice retains its principle taste and strength.
The concern here is that by adding 1/3 water to the grape juice it will lose its principle taste, and will no longer be suitable for the HaGafen blessing. Therefore, you should refrain from doing this.
See Shut Minchas Shlomo no. 4, who is particularly stringent concerning adding water to grape juice; a similar ruling has been given by Rav Elyashiv z”l.