Why "Vayhi" Means Trouble Is Coming | Megillas Esther Explained (1:1)
From the book Megillas Esther Explained (Mosaica Press)
Source: From the commentary on Esther (1:1) in my soon to be published book entitled Megillas Esther Explained by Yisroel Dov Levene (Mosaica Press).

(א) וַיְהִ֖י בִּימֵ֣י אֲחַשְׁוֵר֑וֹשׁ ה֣וּא אֲחַשְׁוֵר֗וֹשׁ הַמֹּלֵךְ֙ מֵהֹ֣דּוּ וְעַד־כּ֔וּשׁ שֶׁ֛בַע וְעֶשְׂרִ֥ים וּמֵאָ֖ה מְדִינָֽה׃

(1) It happened in the days of Ahasuerus—that Ahasuerus who reigned over a hundred and twenty-seven provinces from India to Cush.

The first of something is like an introduction for what comes after. This is certainly true of the possuk in the Torah and even the first word of בראשית – like a seed or DNA that contains all the future within itself וַיְהִי בִּימֵי אֲחַשְׁוֵרוֹשׁ
– It was in the days of Achashverosh. The first possuk here introduces us to one of the main characters of the Purim story.
It says in Pirkei Avos that there is no man that does not have a time when he is successful:

(ג) הוּא הָיָה אוֹמֵר, אַל תְּהִי בָז לְכָל אָדָם, וְאַל תְּהִי מַפְלִיג לְכָל דָּבָר, שֶׁאֵין לְךָ אָדָם שֶׁאֵין לוֹ שָׁעָה וְאֵין לְךָ דָבָר שֶׁאֵין לוֹ מָקוֹם:

(3) He used to say: do not despise any man, and do not discriminate against anything, for there is no man that has not his hour, and there is no thing that has not its place.

We also find in the Gemora in Megilla 6b that when a person is on a winning streak, to be careful of him.

וְאָמַר רַבִּי יִצְחָק: אִם רָאִיתָ רָשָׁע שֶׁהַשָּׁעָה מְשַׂחֶקֶת לוֹ — אַל תִּתְגָּרֶה בּוֹ, שֶׁנֶּאֱמַר: ״אַל תִּתְחַר בַּמְּרֵעִים״. וְלֹא עוֹד, אֶלָּא שֶׁדְּרָכָיו מַצְלִיחִין, שֶׁנֶּאֱמַר: ״יָחִילוּ דְּרָכָיו בְּכׇל עֵת״. וְלֹא עוֹד, אֶלָּא שֶׁזּוֹכֶה בַּדִּין, שֶׁנֶּאֱמַר: ״מָרוֹם מִשְׁפָּטֶיךָ מִנֶּגְדּוֹ״. וְלֹא עוֹד, אֶלָּא שֶׁרוֹאֶה בְּשׂוֹנְאָיו, שֶׁנֶּאֱמַר: ״כׇּל צוֹרְרָיו יָפִיחַ בָּהֶם״.

And Rabbi Yitzḥak also said: If you see a wicked man whom the hour is smiling upon, i.e., who is enjoying good fortune, do not provoke him, as it is stated: “Contend not with evildoers” (Psalms 37:1). And not only that, but if you provoke him, his undertakings will be successful, as it is stated: “His ways prosper at all times” (Psalms 10:5). And not only that, but even if he is brought to court, he emerges victorious in judgment, as it is stated: “Your judgments are far above him” (Psalms 10:5), as though the trial is far removed from him and does not affect him. And not only that, but he will see his enemies fall, as it is stated: “As for all his enemies, he hisses at them” (Psalms 10:5).

In this Megilla we find how king Achashverosh achieved success (and the same later applied to Haman and then switched to Mordechai and Esther). Achashverosh’s success came in stages as seen in the first possuk.
The Gemora in Megila (10b) says:

״וַיְהִי בִּימֵי אֲחַשְׁוֵרוֹשׁ״, אָמַר רַבִּי לֵוִי וְאִיתֵּימָא רַבִּי יוֹנָתָן: דָּבָר זֶה מָסוֹרֶת בְּיָדֵינוּ מֵאַנְשֵׁי כְּנֶסֶת הַגְּדוֹלָה, כׇּל מָקוֹם שֶׁנֶּאֱמַר ״וַיְהִי״ אֵינוֹ אֶלָּא לְשׁוֹן צַעַר.

§ The Gemara returns to the primary topic of this chapter, the book of Esther. The Gemara cites various aggadic interpretations of the verses of the Megilla. The opening verse of the Megilla states: “And it came to pass [vayhi] in the days of Ahasuerus” (Esther 1:1). Rabbi Levi said, and some say that it was Rabbi Yonatan who said: This matter is a tradition that we received from the members of the Great Assembly. Anywhere that the word vayhi is stated, it is an ominous term indicating nothing other than impending grief, as if the word were a contraction of the words vai and hi, meaning woe and mourning.

The sorrow here is הוּא אֲחַשְׁוֵרוֹשׁ – Rashi here comments:

הוּא אֲחַשְׁוֵרוֹשׁ. הוּא בְרִשְׁעוֹ מִתְּחִלָּתוֹ וְעַד סוֹפוֹ:

He was [the] Achashveirosh. He was equally wicked from beginning to end.

He was a constant pursuer of wealth, honor and fame. הַמֹּלֵךְ – he reigned, he became king through buying this position with his wealth. It started by him becoming king over one country. He then extended this to becoming king over another province מֵהֹדּוּ וְעַד־כּוּשׁ– from Hoidu to Kush, two provinces next to each other. This then expanded to שֶׁבַע - seven provinces and then וְעֶשְׂרִים - another twenty, and then וּמֵאָה מְדִינָה - a further one hundred provinces, making him king over the entire world!

תָּנוּ רַבָּנַן: שְׁלֹשָׁה מָלְכוּ בַּכִּיפָּה, וְאֵלּוּ הֵן: אַחְאָב וַאֲחַשְׁוֵרוֹשׁ וּנְבוּכַדְנֶצַּר.

§ Apropos the discussion of the kingdoms of Ahasuerus and Solomon, the Gemara cites a baraita in which the Sages taught: Three men ruled over the entire world, and they were Ahab, and Ahasuerus, and Nebuchadnezzar.

If you enjoyed this, there is the whole passuk by passuk commentary on Megillas Esther, soon to be published entitled Megillas Esther Explained (Mosaica Press).