> VTSAX would show better performance because it includes money made from lending out securities
Correct, VTSAX would probably perform better by a small margin, but it's also hard to compare given that everyone rolls their own "Total Market Index" with slightly different stock composition. At the very least, you're getting more of your money back to you.
> how do these funds decide what percentage to return back to the fund vs keep?
Depending on how much money they want to make :)
vanguard is actually a mutually owned firmed, the only one. So they have little incentive to tax themselves. When you buy vanguard funds, you are also buying ownership of vanguard.
> Why wouldn't Fidelity keep 100% of these fees, and then charge a negative fee?
Because people have no idea how any of this works and "0% expense" rings out. They can make more money this way.