From the aforementioned Gemara (אפילו יחיד המתפלל בער ב שבת) it would seem that the optimum time to recite “Vayechulu” is during davening. However, the Rambam in his Seder Tefillah [Order of Prayer] makes no mention of reciting “Vayechulu” at all. Rather, he mentions it in Hilchos Kiddush. The Rambam writes (Hilchos Shabbos 29:7): “The widespread custom throughout Klal Yisroel is to first recite “Vayechulu”, then to recite a berachah on wine, and then the berachah of “Mekadesh”. The Maggid Mishnah writes: The Rambam’s source is the aforementioned Gemara which says that even individuals should recite “Vayechulu”, consequently, it was established that it should be said over a cup of wine (על הכוס).
However, the Rosh (Shabbos 16:5) writes: “Since even individuals must recite “Vayechulu”, it was established that it should be said in the middle of shemonah esrei. That which we say it after shemonah esrei as well, is because of Yom Tov that falls on Shabbos where we don’t say it in the silent shemonah esrei and in order not to differentiate from one Shabbos to the next we say it every single Shabbos.” The Tur (268) adds an additional reason for saying it after the silent shemonah esrei, which is to benefit those present who may not know how to say it.
The Rosh adds: In the Seder Tefillah of the Kadmonim we don’t find that “Vayechulu” was said as part of davening. This is like the Rambam we mentioned above, which makes no mention in his Seder Tefillah of saying “Vayechulu”. However, the Ohr Zoruah (Hilchos Erev Shabbos siman 20) writes: Those who don’t say “Vayechulu” in shemonah esrei are simply mistaken.
Saying “Vayechulu” During Kiddush
The Rosh adds: The minhag of saying “Vayechulu” by kiddush is for the benefit of those who may not yet have heard or recited “Vayechulu”, as is often the case by women and children who don’t normally daven (see Tosfos Pesochim 106a). According to this, if everyone has davened there would be no need to say “Vayechulu”. However, the Chasam Sofer (Shu”t, Kovetz Teshuvos 10) was asked about someone who has no wine and makes kiddush on bread, if it is ok to interrupt between the berachah of “al netilas yodayim” and “hamotzi” with the recital of “Vayechulu”, and he writes: Since it is part of the nussach [text] of kiddush, it’s not considered an interruption (see also Levush 272:9 who writes this). Based on this, even if one doesn’t need to be moitzi anyone [say it for others who haven’t yet heard or recited it] he should still mention “Vayechulu” in kiddush as it is part of the standard nussach.
Even without this, there is a special inyan [idea] of saying “Vayechulu” three times as is clear from the Mishnah Berurah (271:45).
What If One Is Delayed and Only Recites Kiddush Late at Night, or Shabbos Morning?
However, the Rema (271:8) writes: “If one never made kiddush at night, he should make it up the next day, however, he should omit Vayechulu”. The question is, if “Vayechulu” is part of the nussach of kiddush, why in such a case is it omitted?
The Mishnah Berurah (s.k. 40) explains: The Creation of the World was completed by Friday night, therefore, there is no point saying it after that time.
Practically, if one is ill when Shabbos comes in, and he only gets around to saying kiddush by the end of the night, although the Levush (sif 8) writes not to say “Vayechulu”, the Pri Megodim (Mishbetzois Zahav s.k. 11) rules that one should say it. The Toras Shabbos (271:14) explains: Although creation of the world was completed at the beginning of the night, since the earth is round, and the time of the beginning of night is different in all places, one can say “Vayechulu” the entire night. Others explain: The reason one may say it the entire night, is because the entire night is considered one (see Aruch HaShulchan sif 21).