Saying Vayechulu after Shemon eh Esrei on Friday nights-How & Why:
The start of the custom and its purpose and reason: The saying of Vayechulu after Shemoneh Esrei of Friday night was accustomed due to the Yom Tov Friday night Davening in which Vayechulu is omitted from the Yom Tov Shemoneh Esrei, and is hence required to be said afterwards. It then became accustomed to be recited every Friday night in order not to differentiate between one Shabbos and the other. Likewise, it was instituted to be said in order to be Motzi one who does not know how to say it [and thus did not say it in Shemoneh Esrei].
Saying it standing and in unison: It is customary to recite Vayechulu in unison, the entire congregation together, in a loud voice and in a standing position. The reason for this is because through its recital we testify to G-d regarding the creation, and witnesses need to stand during their testimony.
 Admur 286:12; Michaber 268:7; Tur 268:7; Rosh Brachos 10:15; Tosafus Pesachim 106a
 Admur ibid; Michaber ibid; Tur ibid; Levush 268:7; Hamanhig Shabbos 5
 Admur ibid; M”A 268:10; Taz 268:5; Tur ibid; M”B 268:19; Biur Halacha 268:7 “Umeumad”
The reason it is said together in unison: As some Talmudic Sages hold that testimony needs to be said in unison by the witnesses. [See Tana Kama Sanhedrin 30]. Now, although we do not rule like this opinion [see Michaber C.M. 28:9], nonetheless regarding the testimony of Maashe Bereishis, we do like the Tana Kama and testify all together. [See M”A ibid and Machatzis Hashekel ibid and Levushei Serud]
 Admur ibid; Michaber ibid; Tur ibid
 Admur ibid; Taz ibid; Tur ibid; M”B 268:19; See Michaber C.M. 28:5