Chance the Rapper was the first guest on “The Vic Mensa Show,” which premiered Wednesday, and about an hour and a half in, Mensa brought up “Letters.” “You, as one of the most Christian [rappers] of all time, to have a scathing critique of the Black church — I thought that was a real interesting angle,” Mensa said. Chance traced it back to a phone call. Right before his first trip to Ghana, his grandmother called to tell him where to go and who …