Rabbotai,
The story of Yanai Hamelech& the chachamim used the word "Vayomer" several times in the narrative. That form, (vov hahipuch - vayomer -and he said), is Tanach loshon. I can't remember the Mishne or Gemara using it elsewhere.
Why was it used here davka?
B'kavod,
Samuel Kosofsky
You bring up a good point. I have not found an answer, but I can suggest an answer "Al Pi Drush." The Gemara in Beitzah (16a) Darshens the word "va'Yinafash" (Shemos 31:17) as "Vay she'Avdah Nefesh." This shows that the Vav-Yud combination can sometimes be used to mean sadness. It is possible that by using such terminology where the "Vay" is constantly repeated, the Tana was trying to connote the sadness of this event, where one evil Jew led to the killing of all of the Chachmei Yisrael. Similarly, it could have been written in a Tanach Lashon to show that the tragedy was of epic proportions, like many of the tragedies in Tanach.
Kol Tuv,
Yaakov Montrose