If someone recited borei pri hagefen on Shabbos night and forgot to say vayechulu, should he go back and say vayechulu or just continue with the rest of kiddush, drink the wine and then recite vayechulu later?
If one forgot to say Vayechulu, and began the berachah of Borei Peri Ha-Gafen, one should carry on until the end of Kiddush, and drink the wine.
After this, another cup of wine is filled, and Vayechulu is recited separately on the second cup of wine.
See Shulchan Aruch Ha-Rav (271:19), who explains (based on the Magen Avraham 21) that aside from being recited in shul, there is an obligation to recited Vayechulu at home upon a cup of wine, in order to be motzi the family.
He continues that therefore if Vayechulu was forgotten, it should be recited during the meal over a cup of wine.
It is noteworthy that the Magen Avraham cites this halachah in the name of the Mateh Moshe, who cites from the Rokeach. However, the actual halachah of the Rokeach (no. 7) is somewhat different, discussing a case of somebody who forgot to say Vayechulu rather than forgetting during Kiddush. The Eliyah Rabba (19) points out this problem.
Nevertheless, the halachah follows the ruling of the Magen Avraham and the Shulchan Aruch Ha-Rav, and if Vayechulu is forgotten during Kiddush, it should be recited again after Kiddush.
Because it is not a full part of Kiddush, it cannot be said in between the berachah of Borei Peri Ha-Gafen and drinking the wine, and it is therefore recited later.