The story
Vṛndā was faithful to her husband Jalandhara, and that faithfulness made him unbeatable. Viṣṇu took his form to break it, and when she understood she cursed him to become a stone. He accepted the curse, became the śāligrāma, and gave her the tulasī plant as her form so that no offering to him would ever be complete without her.
The marriage today is that promise kept. The plant in the courtyard is married to Viṣṇu with the full rite, and the household gives its own daughter away.
It is why a tulasī leaf sits on every Vaiṣṇava offering, and why the plant stands in a raised, decorated pot rather than in the ground.