Shadow vines can be obtained in Herblore Habitat. They are obtained by tracking Shadow jadinkos in Herblore Habitat. Shadow vines are the secondary ingredient of the Scentless potion. It requires a herblore level of 59 to make it and a clean argway herb.
In order to obtain a shadow vine, you must have a red vine blossom, and an abandoned house habitat. They are caught via tracking, which requires a noose wand. The area for tracking these is located on the southernmost part.