Can Someone Explain To Me How Vayne Won't Be Permanently Invisible?

I've never really played her so maybe I just don't know how this works...

Her rank 5 Tumble is a 2 second cooldown and grants 1 second of invisibility. Now, I'm not very good at math but I'm pretty sure 50% of 2 seconds is 1 second...

Like, are we supposed to target her in the brief moment she uses the empowered auto attack from her tumble before she tumbles again? Because that doesn't work with Akali.

Is Vayne just going to be able to literally dodge roll down a lane in Invisibility like an assassin?

I'm just confused.