Rabbi Yonasan Eubshitz gives a very interesting interpretation on vaadoni zoken, which refers to Hashem. When an open neis is being done like yetzias mitzraim, Hashem revealed Himself as a gibor, general but at kabolas hatorah as a zaken because no open neis or miracle was done there. Sarah Imenu saw Hashem as a zaken indicating to her that here no miracle will be done and she cannot have a child as she did not have a womb, so vaani zakanti refers to Hashem. Lo tzochakti, I did not laugh for not being possible, but ki Yarea is a yiras horamemus feeling that she is not worthy to this type of miracle.
