Where is Tza'ir, and why did Yehoram go there?
Radak: It is a city in Edom. He went to fight Melech Edom.
Why does it say "va'Yhi Hu Kam Laylah"?
Radak: When Yehoram was in Tza'ir, troops of Edom came out and camped opposite each other, until they would fight tomorrow. During the night, Yehoram and his camp rose and struck Machaneh Edom around him, and struck Sarei Rechev. When Machaneh Edom realized, they were befuddled, for they came upon them suddenly.
Radak citing his father: Melech Edom rose at night and struck Edom around Melech Yehudah, i.e. those who helped him and did not rebel yet. Melech Edom struck them and Sarei Rechev of Yehoram.
Malbim: Even though Yehoram went with all his officers and chariots (Divrei ha'Yamim II, 21:9), he was so weak that he was afraid to fight them during the day.
What is the meaning of "ha'Soveiv Elav"?
Rashi: It is close to the border [of Yisrael].
Radak citing his father: It is Bnei Edom who had not rebelled and still supported Melech Yehudah.
Malbim: He struck only those around Yerushalayim.
Why is there a Tzeirei under the Veis in "ha'Soveiv"?
Radak: This is unusual, but we find like this - "Hakeif Es ha'Ir" (Yehoshua 6:3), "u'Va'al Kenafayim Yaged Davar" (Koheles 10:20), "v'Holeid Aven" (Yeshayah 59:4).
Who fled?
Radak: The remainder of Edom fled to their tents.
Radak citing his father, Malbim: Yisrael fled to their tents.
