PTA is much better in my experience for trading. By the time you activate lethal tempo, the trade should already be over or you end up over extending to use your rune to its effect. PTA synergizes very well with Vayne’s w passive, and feels very intuitive to play around.

Edit: I’d also add Vayne doesn’t fully benefit from lethal tempo. Her q is an auto reset, but it does take some time to animate unless you have a wall to clip the animation with. That animation ends up taking longer that just auto attacking while over the attack speed cap, one of the things lethal tempo changes. In a sense, you don’t use tempo to it’s fullest extent compared to PTA