Vine is used mainly to connect Amaterasu or Chibiterasu to a Konohana blossom (「桃コノハナ」?; Momo Ko no hana) so that they may progress on their journey. When a Konohana Blossom is upside-down, another vine may be drawn while in mid-air to the next Blossom. In Ōkamiden, a line may also be drawn connecting Chibiterasu and his partner in order to move Chibiterasu to his partner's location, such as across water or over a gap.
Vine is also sometimes used to pull objects together to solve a puzzle. In Ōkami, this is usually done by connecting a Konohana Blossom and a hook (such as those on the Spider Queen's abdomen), while in Ōkamiden, this is typically done by connecting two hooks.
In Ōkami, a line is drawn from the Konohana Blossom to the desired object, whether Amaterasu or hook.
In Ōkamiden, however, the line is drawn from Chibiterasu to the Konohana Blossom or his partner. This is likely to avoid the conflicts with Guidance which would occur if the line was drawn from the target back to Chibiterasu. In the case of hooks, either side may be used to begin the line.
Vine is obtained inside the statue of Tsuta Ruins, an area which cannot be entered until the water is purified. Inside the statue is a grassy area with numerous minor areas of cursed grass. When the grass is cleansed of the curse, a constellation will appear, and Tsutagami will grant Amaterasu Vine.
Vine is obtained inside a large rock protruding into a room in the Five-Story Pagoda. When the rock is opened using Cherry Bomb, a large opening is found inside, along with a statue of Tsutagami. When the statue is interacted with, a constellation will appear, and Tsutagami's child will grant Chibiterasu Vine.
Spoilers end here.
When Vine is used,the kanji 結 ("musubu", "tie") appears.