How did Vishnu help Indra in killing Vritra?
And Vishnu said, 'I am no doubt bound to do what is for your good. I shall, therefore, tell you of a contrivance whereby he may be annihilated. Do ye with the Rishis and the Gandharvas repair to the place where Vritra that bearer of a universal form is and adopt towards him a conciliatory policy. You will thus succeed in overthrowing him. By virtue of my power, victory, ye gods, will be won by Indra, for, remaining invisible, I shall enter into his thunderbolt, that best of weapons. O foremost of gods, depart ye with the Rishis and the Gandharvas. Let there be no delay in effecting a peace between Indra and Vritra.'
Vishnu was clearly an active participant in the killing of Vrtra.
Why did Brahmahatya attach only to Indra for killing Vrtra?