There was an evil spirit child of Lord Shiva whose name was Jalandhar. He was an amazing evil presence and was causing decimation in everywhere on the world. His significant other was Vrinda who was a major fan of Lord Vishnu. Vrinda was an upright and loyal spouse. Because of Vrinda's loyalty, Jalandhar couldn't overcome or be murdered as her steadfastness made a shield on him. To murder Jalandhar, Vrinda's dependability must be broken.
Thus, to take care of this work, Lord Vishnu appeared as Jalandhar and went to Vrinda. Trusting Lord Vishnu as her significant other, the steadfastness of Vrinda got broken and the shield got broke. From that point onward, Jalandhar was slaughtered.
Knowing this, Vrinda reviled Lord Vishnu of taking the human structure and feeling a similar agony as she was enduring of her life partner's misfortune. Master Vishnu acknowledged her revile and gave her the type of tulsi, the plant and announced her the holiest plant and to raise her pride, he wedded to her and acknowledged her as his significant other.
Afterward, because of the scourge of Vrinda, Lord Vishnu took the human type of Lord Ram and felt the enduring of her life partner misfortune when Raavan removed Sita from him.
This is the reason Tulsi is hitched to Lord Vishnu.