Why does it mention planting vineyards in Harei Shomron?
Radak: People normally plant vineyards in mountains, so the wine will be good. When they feared enemies in their land, they feared to go out to vineyards, and all the more so to plant - "ba'Chitzim uva'Keshes Yavo Shamah" (Yeshayah 7:24). Now He promises them that they will have great serenity, and they will plant vineyards. Also redemption demonstrates serenity; it would not be done at the time of fear. 1 It mentions Shomron, for there they feared the enemy many days amidst dire siege.
Malbim: It says "v'Samti Shomron l'Iy ha'Sadeh l'Mata'ei Charem" (Michah 1:6). Also refer to 31:4:3:3.
Why does it say "Nat'u Not'im"?
Radak: The past tense is used in place of future.
Malbim: The Kusim that Sancheriv settled there planted [trees] there. You will plant vineyards there.
What is the meaning of "v'Chilelu"?
Rashi: In the fourth year you will redeem the fruits (Kerem Revai 1 ) onto money.
Radak: And redeem the fruits in the fourth year like the law of redemption of fruits of a vineyard, and make them Chulin, and then a Zar 2 may eat them. It mentions Chilul, for it was done with Simchah and song - "Kodesh Hilulim" (Vayikra 19:24).
Malbim: People of Shomron redeemed their vineyards and did not take the fruits to Yerushalayim [because Yaravam and the other Malchei Yisrael after him blocked people from ascending to Yerushalayim for the festivals].
A Tana holds that the Torah law of Revai applies only to vines, but not to tree fruits. Our verse implies that it applies also to trees! This supports the opinion in Tosfos (Kidushin 2b) that even that Tana agrees that mid'Rabanan, Revai applies also to tree fruits. (PF)
Radak's wording is difficult. Even before Pidyon, a Zar may eat them, but only in Yerushalayim, in Taharah! Pidyon permits eating them anywhere, even b'Tum'ah. (PF)