The vine ladder is used to access the middle level of the Runespan from the bottom level. It requires level 33 Runecrafting to use, and is located near the middle of the Runespan.
Temporary boosts can be used to climb the ladder, so it is possible to do so at as low as 26 Runecrafting (with a random Advanced pulse core +7 boost), or 28 Runecrafting (with super runecrafting potion's reliable +5 boost). However, as 26-33 Runecrafting is fairly quick, boosts are generally not worth the effort.
A map of the lower level. The vine ladder is on the island numbered '51'.