To my knowledge, Reddit is owned by private companies and investors. Blackrock and Vanguard have no ownership stake, or a very small and very indirect ownership stake.
For what it’s worth, a significant percentage of every (reasonably liquid) public company on Earth is owned by Vanguard and Blackrock, because those companies manage trillions of dollars in assets (many of which are middle-class people’s retirement investments). They aren’t a conspiracy. They’re asset managers, and mostly passive managers at that.