Whom does the verse address?
Radak: It is a command to the enemy [to come and destroy].
What are "Saroseha"?
Rashi #1: They are plains. Refer to 5:10:5:1-2.
Rashi citing Menachem, Targum Yonasan: They are walls (Radak - of Yerushalayim], like "Adaleg Shor" (Tehilim 18:30). Refer to 5:10:5:1-2.
Rashi: They are rows of a vineyard.
Rashi citing Targum Yonasan: Its cities.
What should not be finished off?
Radak: It does not apply to the wall, for it was totally destroyed - "v'Es Kol Chomos Yerushalayim Saviv Nastzu" (52:14)! Rather, do not totally finish off the Am.
What are "Netishoseha"?
Rashi: They are vine roots.
Rashi citing Targum Yonasan: They are mansions.
Radak: They are teeth (protrusions) of the wall.
Radak: Some say that this is like "v'Es ha'Netishos Hesir Hesaz" (Yeshayah 18:5). They are the branches that spread out 1 . Malbim - they spread out of the vineyard and weaken the vines.
They must hold that Saroseha are rows of a vineyard, like Rashi writes; the branches are relevant to it. However, Radak brought only one Perush of Saroseha, i.e. walls! (PF)
Why does it say "Lo la'Shem Hemah"?
Radak: They were of Hashem - "v'Ganosi Al ha'Ir ha'Zos Lehoshi'ah [Lema'ani]" (Yeshayah 37:35). Now, the walls are not of Hashem to protect them. The city abandoned Him, and He abandoned it to the enemy.
Malbim: The vineyard itself (Yisrael) is of Hashem; remove the Resha'im 1 - "v'Es ha'Netishos Hesir Hesaz" (Yeshayah 18:5).
Malbim (Yeshayah 18:5): The wayward branches are the 10 tribes, that were grafted onto (mixed with) Nochrim. They will be cut off, i.e. exiled to another land. (This implies that they mixed with Nochrim borrower they were exiled. I do not know a source for this. - PF)
QUESTIONS ON RASHI
Rashi writes that Netishoseha are vine roots. Why are these mentioned with Saroseha, whether the latter are plains or walls?
Rashi: Rather, Saroseha are rows of a vineyard.
Rashi citing Targum Yonasan: Netishoseha are mansions; Saroseha are cities.
