In Star Wars Jedi: Fallen Order, there's no official content indicating that Darth Vader would specifically force Cal Kestis to kill Cere Junda if Cal were to become an Inquisitor wielding a red lightsaber. The game's narrative primarily focuses on Cal's journey as he grapples with the light and dark sides of the Force, as well as his efforts to rebuild the Jedi Order while being pursued by the Empire.
While Vader's methods can be manipulative and he uses fear to control individuals, the game’s storyline does not present a scenario where Cal is directly commanded by Vader to kill Cere. Instead, the narrative emphasizes themes of choice, redemption, and the consequences of actions taken in the face of dark temptations.
If you have specific scenarios in mind or fan theories, those can certainly be explored, but within the established canon of the game, this particular encounter does not occur.