Culpeper places vervain under Venus, and Libra receives it in the standard tables because Libra is Venus's air sign. Both steps deserve separating.
The Venus attribution is Culpeper's own. It does not come from Pliny, from Roman cult, or from any classical source; the ancient material establishes that vervain was sacred, not that it was Venusian. Culpeper's grounds are his usual ones, the plant's use for the nerves, headache and the cleansing of the blood, fitted into his scheme.
The step from Venus to Libra is not Culpeper's at all. He assigns plants to planets and only occasionally to signs, and vervain is not given a sign in the Complete Herbal. The zodiacal placement is a later editor's derivation by rulership: if the plant is Venus's and Venus rules Libra, the plant is Libra's. That inference is mechanical and it is worth knowing that it is an inference, because it is printed everywhere as though it were a separate tradition with independent authority. It is not; it is arithmetic performed on Culpeper.
This is the general shape of the problem across the whole plant-correspondence literature. There is usually one attribution with a name attached to it, and then a superstructure of derived placements (signs, elements, tarot cards, chakras, angels), generated by applying rules to that single attribution and then presented as a body of ancient lore. Naming which step is the sourced one, and which are derived, is most of the useful work.
Modern herbalists use vervain principally as a nervine and bitter. Its constituents include verbenalin and verbascoside.