What is the story behind Tulsi Vivah — why is this plant married to Lord Vishnu?

The origins of Tulsi Vivah lie in the story of Vrinda, a supremely devoted wife who was deceived by Lord Vishnu (in disguise) in order to defeat the demon Jalandhar. Feeling betrayed, Vrinda cursed the Lord to take the form of a stone (Shaligrama) and transformed herself into the Tulsi plant. Out of compassion and to honour her unparalleled devotion, Lord Vishnu declared that he would forever be worshipped alongside Tulsi, and that the Tulsi Vivah would be observed to celebrate their eternal spiritual union.