The core dispute is already visible in Shulchan Arukh, Orach Chayim 271:10 itself: the base ruling records that one recites Vayechulu standing — because it is testimony about creation — and then proceeds to kiddush, while the Rema's gloss immediately qualifies that one *may* stand during kiddush but it is *preferable to sit* (citing the Kol Bo).
Mishnah Berurah 271 explains both sides of the tension: sitting is preferred because it constitutes greater "kiddush in the place of the meal" (kiddush bi-mekom seudah), and, following the Vilna Gaon's reasoning, because when one person exempts others they must establish a shared setting — and sitting constitutes that fixed, joint presence; accordingly, the listeners too should ideally be seated.
A competing practice pulls in the opposite direction: the Shelah (Aseret HaDibrot, Shabbat, Ner Mitzva 115) records the custom of standing for Havdalah despite the general principle that sitting is required when one person fulfills the obligation on behalf of others, justifying standing on the grounds that it reflects the manner of escorting — suggesting that for some authorities a posture of honor or witness overrides the seated-assembly rationale.