What about Vashti did Achashverosh remember?
Rashi: Her beauty; therefore, he was sad.
Malbim: After his anger subsided and he thought to take another wife, he realized that it will be hard to find someone with her beauty, lineage and attributes. Also refer to 2:1:2:2.
Why does it say "Achashverosh remembered? what she did and what was decreed upon her"?
Torah Temimah citing Megilah 12b: Hashem treats people Midah k'Neged Midah. We derive that Vashti the Resha'ah used to strip Jewish women of their clothing and make them work on Shabbos. Just like she did (make women naked on Shabbos) was decreed upon her 1 .
Malbim: (a) He remembered what she did, and feared to take another wife, lest she have lineage and beauty like Vashti, and do like Vashti's deed. If he will kill also the new wife, people will say that he kills his wives, and his throne of Mishpat will not be founded. (b) He remembered what was decreed upon her, and feared that even if he finds a proper wife, she will not agree to marry him, amidst fear lest he kill also her.
Vilna Gaon: "Nigzar" implies from Shamayim, even though it is not proper for her. Therefore, the Gemara expounds, due to what she did to Yisrael.
