How did Ezra save Ahsoka in "World Between Worlds"?

Vader doesn't necessarily see anything. At the time he swung the floor was collapsing below him, the air was full of dust, there were bright energy discharges, and then he goes through the floor as he swings. Given that we've seen how effortlessly lightsabers slice through people, he could have easily assumed he'd struck and killed her, and her body was lost in the collapse (which, as he's seen limping away later, he clearly had a rough time dealing with). Since she'd been pulled through time, there'd be nothing for Vader to sense; she was just gone, and, he could have assumed, therefore dead.