How is it different from Bitcoin conceptually? Bitcoin reflects financial markets overall just as well as any Vanguard-managed fund. It does not matter that Vanguard intention is precisely that while Bitcoin's isn't if the outcome is the same.
Just in case I'll explain what I mean by an analogy: Imagine we are sailing an ocean in a small ship, and consequentially the ship rocks really hard. There's a table with two straight sticks on it in everyone's room, and the sticks constantly slide around the table due to rocking. And for whatever reason the parent comment author is tasked with keeping them as parallel as possible, while not stopping their movement alltogether. To do so he makes a contraption with electric motors attached to one end of each stick and sensors that detect when sticks are not parallel, which causes motors to align them. In the mean time in my room I took my sticks from the table and attached them to the ceiling instead because they look nice there. Here despite the lack of intention on my part my sticks will tend to stay parallel without much effort.