If Vishnu was the 'Supreme Personality of Godhead', why did he have to rape his sincere devotee Vrinda to defeat Jalandhar who was merely a demon who did not achieve any powerful boons? If Vishnu was God then he could have destroyed Jalandhar just by willing so.
Vishnu was not "God." He was a god. Hinduism has a variety of labels on what kind of religion it is (i.e. polytheism, monotheism, atheism, non-theism...). But for the most part, more then one being is labeled as a god.
Vrinda was married to King Jalandhar. And because she was so devoted to Vishun, Jalandhar became invincible to the point not even Shiva could defeat him. Shiva asked Vishun to find an answer. His solution was to claim Vrinda's chastity, thus resulting in Jalandhar's loss of power. Shiva killed him.
The thing is a little complicated, but it was the only possible way Lord Vishnu had.
Jalandhar was a demon king and his wife Vrindha was a great devotee of Lorn Vishnu.
Once upon a time, Jalandhar got a boon from Lord Shiva that he will be invincible and undefeatable till his wife is faithful to him.
The demon king after receiving the boon started fighting and defeating all the gods one by one.
He even started fighting Lord Shiva, but due to the boon, Lord Shiva wasn't able to kill him.
Now, Lord Vishnu had only one way to help Devas i.e by taking Vrinda's chastity. Lord Vishnu disguised himself as Jalandhara and took Vrinda's chastity. By doing this, Jalandhar became powerless and was easily defeated by Lord Shiva.
On knowing that she was cheated by her god, Vrinda cursed Lord Vishnu that he'll become a stone Shaligram. Later Vrinda ended her life by offering herself to fire.
Lord Vishnu knew that he had committed a great sin, so he accepted his fate. But to make sure that Vrinda would not suffer, he gave her a boon that after she's dead, a plant will grow from her ashes Tulsi and in Hindu rituals, Tulsi will be married to Shaligram, thus confirming that Shaligram and Tulsi are husband and wife.
Also, Lord Vishnu's prasad isn't served without Tulsi leaves in it.