The work Ta’alumot Leb (vol. 3, 19:4) discusses the situation of a Berit Mila that takes place in the synagogue on Shabbat morning. It is customary at a Berit Mila for the Rabbi to recite the Beracha of "Boreh Peri Ha’gefen" and the special Beracha for the Berit over a cup of wine. On Shabbat morning, however, one may not eat or drink anything before reciting or hearing Kiddush, and the custom on Shabbat is to conduct a Berit immediately after the prayer service. How, then, can the Rabbi drink the cup of wine at the Berit, if he had yet to recite or hear Kiddush?
The Ta’alumot Leb records the practice of a certain Rabbi who, in such situations, would ensure to drink a full Rebi’it (approximately 3.2 oz.) of wine from the wine of the Mila, and have in mind to fulfill the Misva of Kiddush with this cup of wine. In order to fulfill the Misva of Kiddush, one must hear or recite Kiddush "Be’makom Se’uda" – at a place where he then eats. This Rabbi would therefore make a point of drinking a Rebi’it of wine from the cup of the Berit Mila, so that he can fulfill the requirement of reciting Kiddush at the place where he then drinks wine.
This practice demonstrates two important Halachot. First, it shows that one can use one cup of wine for two different Misvot. The Rabbi in this situation used the cup of wine for both the Berachot over the Berit Mila as well as for Kiddush. Although there is a rule of "En Osin Misvot Habilot Habilot," which forbids "bunching" different Misvot together, this rule applies only to performing two different Misva acts simultaneously. For example, during the times of the Mikdash, it would be forbidden for a Kohen to give the special Sota waters to two women at the same time. However, one may perform a single Misva act to fulfill two different requirements. Thus, one may use the cup of wine at a Berit Mila for Kiddush, as well.
The second important Halacha reflected by this practice is that drinking a Rebi’it of wine suffices to fulfill the requirement of "Kiddush Be’makom Se’uda." A person who recites Kiddush on Shabbat morning fulfills his obligation even if he does not eat anything at the site of the Kiddush, provided that he drinks at least a Rebi’it of wine. It often happens that a Rabbi attends a Kiddush on Shabbat morning where he is invited to recite Kiddush on behalf of everyone assembled, but he does not intend to stay to eat. In such a situation, he should simply ensure to drink at least 3.2 ounces of the Kiddush wine, and he is thereby considered to have recited Kiddush "Be’makom Se’uda." This is the ruling of numerous Halachic authorities, including Maran (in Bet Yosef), the Rama, the Peri Hadash, the Hid"a, Rav Yaakov Kuli (in Yalkut Me’am Lo’ez), and the Shulhan Gabo’ah.
It should be noted that if a person does not drink a Rebi’it of wine, then he does not fulfill the obligation of Kiddush unless he eats a Ke’zayit of baked goods. He does not fulfill his obligation by eating other foods, like fruits, vegetables, potatoes, and even rice.
There is a dispute among the Halachic authorities as to whether one can fulfill the requirement of "Kiddush Be’makom Se’uda" by drinking a Rebi’it of grape juice. Rav Yosef Shalom Elyashiv (contemporary) is reported to have raised the question and leaned toward the stringent position, that grape juice resembles grapes in this regard, and it therefore does not suffice for the requirement of "Kiddush Be’makom Se’uda." Hacham Ovadia Yosef, however, after citing Rav Elyashiv’s ruling, writes that he disagrees (listen to audio recording for precise citation). He maintained that grape juice is considered the same as wine, and thus one may fulfill the obligation of Kiddush on Shabbat morning by drinking a Rebi’it of grape juice, even if he does not eat anything.
Summary: In order to fulfill the obligation of Kiddush on Shabbat morning, one must either eat a Ke’zayit of baked grain products or drink a Rebi’it (approx. 3.2 oz.) of wine or grape juice after reciting or hearing Kiddush. When a Berit Mila is held in the synagogue on Shabbat morning after the prayer service, the Rabbi should ensure to drink a full Rebi’it of wine from the cup of the Berit Mila and have in mind to thereby fulfill the Misva of Kiddush.
