Can a yahid recite “Vaychulou Hashamaim” in Shabat maariv after shmone esrei?
The opinion of the Mishnah Berurah is that it should be recited specifically with somebody else, as an eidus, testimony to Hashem’s creation of the world.
However, the Chazon Ish is known to have disputed this ruling, and maintained that there is no need to say together with others.
See Yerushalayim Be-Mo’adeha (p. 164), who brings a number of opinions, and see also Shut Avnei Levi, no. 15.
In practice, when somebody is davening alone, he should say Vayechulou without the intention of “giving testimony,” but as part of the prayer service of reciting pesukim.