If the recitation of “Vayechulu” is considered giving eidus, then it would seem that one is obligated to stand, as is clear from the Gemara in Shavuos (30b) and the Rambam (Hilchos Sanhedrin 21:3) that eidus must be given whilst standing, learnt from the pasuk ועמד ו שני האנשים – “The two men should stand”. However, the Rishonim argue as to whether bedieved [post-facto] eidus given whilst sitting down is pasul. The Rashbah and Chiddushei HaRan both bring: “Some say if eidus is given whilst sitting down, the eidus is not considered eidus at all”, however, they both disagree. The Ritva and the Ran (14a MiDafay HaRif) also disagree and learn that the din of standing is lechatchilah [ideal], however, bedieved sitting down is ok. This dispute would seemingly make a practical difference when it comes to kiddush, and whether one would need to repeat “Vayechulu” if it was said whilst sitting down. Unless we differentiate between normal eidus and eidus of “Vayechulu” by kiddush (see below).
Is It Ok to Lean on a Shtender Whilst Reciting “Vayechulu”?
The Shulchan Aruch (268:7) writes: אומרים ויכולו אחרי התפלה מעומד – “We say ‘Vayechulu’ after shemonah esrei whilst standing”. The question is: Is one allowed to lean on shtender in a manner where if the shtender would be taken away one would fall?
The Ishei Yisroel (36:25) cites proof that one is allowed to lean whilst saying “Vayechulu” from a Rema (Choshen Mishpot 17:1) in Hilchos Eidus. The Rema writes: עמידה ע ״ י סמיכה נ קראת עמידה - “Standing whilst leaning is considered standing”, and the Taz adds that even if the thing one is leaning on would be taken away one would fall it’s still ok, therefore, the Ishei Yisroel concludes the same thing is with “Vayechulu”.
However, in Hilchos Eidus many poskim disagree with the Taz and say one isn’t allowed to lean (see Urim VeTumim, R’ Akiva Eiger, Birkay Yosef, Knesses HaGedolah end of siman 28 and Magen Avraham Hilchos Rosh Chodesh end of siman 422). If the recitation of “Vayechulu” isn’t considered proper eidus than there would be room to be lenient, however, practically the Shulchan Aruch rules that it should be recited whilst standing, therefore, it would seem that lechatchilah one shouldn’t lean.
Standing for “Vayechulu” During Kiddush
We mentioned above the Rema (271:10) which says that the minhag is to sit for kiddush, even when saying “Vayechulu”. The Mishnah Berurah (s.k. 47) explains: Since “Vayechulu” was already said in shul whilst standing, there is no need to stand a second time when saying it in kiddush. Although, one is now being moitzi his household members, R’ Moshe in Igros Moshe (Orach Chaim 5:16 s.k. 5) writes, since one already said eidus in shul, it makes no difference when saying it a second time if one is standing or sitting, and in regard to those listening it’s not considered eidus, as the concept of shome’a ke’onah [hearing is like saying] doesn’t work for eidus, see inside for a lengthy discussion about this.
Nonetheless, the Mishnah Berurah (268:19) writes: If one forgot to say it in shul, then when he says it during kiddush he should make sure to stand. He then concludes by quoting the Pri Megodim (Eishel Avraham s.k. 10) who says that the minhag is to stand when reciting “Vayechulu” even if one already said it in shul. This is also how the Kitzur Shulchan Aruch (77:3) and Ben Ish Chai (Year 2, Bereishis 29) take on.
Although, it’s brought down that the Chofetz Chaim (cited in Chayov U’Poalov Vol. 3, pg. 911), R’ Chaim Brisker and the Brisker Rov (cited in Haggadah Shel Pesach MiBeis Levi pg. 92), the Chazon Ish (Dinim V’Hanagos 11:2) and the Steipler (Orchos Rabbeinu Vol. 1 pg. 109) all sat down for the entire duration of kiddush.