It is a biblical commandment to remember the Exodus every day, as the Torah says: “So that you remember the day you left Egypt every day of your life” (Devarim 16:3). The Torah seems to add a superfluous word (“kol yemei ĥayekha” instead of just “yemei ĥayekha”), from which the Sages derive that the mitzva to remember the Exodus is performed both during the day and at night (Berakhot 12b). This mitzva can be fulfilled by reciting any verse that discusses leaving Egypt or by mentioning the Exodus in one’s own words.
There are two reasons why the Va-yomer paragraph was incorporated into the recitation of the Shema. First, it mentions the mitzva of tzitzit that reminds us of all the mitzvot. Second, it discusses the Exodus from Egypt. It is therefore customary to say Va-yomer even at night, for although there is no need to mention the mitzva of tzitzit then, there is still reason to say it for the sake of remembering the Exodus (see Berakhot 14b, and Kessef Mishneh, Laws of Keri’at Shema 1:2-3).
There is a difference between the mitzva of Keri’at Shema and the mitzva of remembering the Exodus from Egypt. The mitzva of Keri’at Shema can only be fulfilled in the first three hours of the day because that is the time we wake up, whereas the daytime mitzva of remembering the Exodus can be performed throughout the entire day. However, following the enactment of the Sages, we fulfill the mitzva of remembering the Exodus by saying Shema. The Exodus is also mentioned in the berakhot of Emet Ve-yatziv in Shaĥarit and Emet Ve-emuna in Ma’ariv, and one who recites them fulfills his obligation of remembering the Exodus even if he did not recite the Shema.
Regarding women, some poskim say that since the mitzva to remember the Exodus from Egypt lasts continuously throughout the day and night, it is not a time-bound mitzva, and women are therefore obligated to fulfill it. Hence, women are required to recite Emet Ve-yatziv in Shaĥarit and Emet Ve-emuna in Ma’ariv (MA). Still, according to the majority of poskim, because there is a specific mitzva to remember during the day and a specific mitzva to remember at night, it is considered a time-bound mitzva from which women are exempt (Sha’agat Aryeh §13; MB 70:2).
Nonetheless, a woman who wishes to perform this mitzva is praiseworthy. It is preferable for her to fulfill this mitzva by reciting Emet Ve-yatziv, because Va-yomer mentions the mitzva of tzitzit, from which women are exempt, whereas Emet Ve-yatziv, which discusses the redemption of Israel, pertains to both men and women. Moreover, if she recites this berakha followed immediately by the Amida, she will have fulfilled the enhancement of juxtaposing redemption and prayer (see below, section 13).