I'd have to say that if they added a whole 2-3 timer to her True Damage procs it would completely gut her as an ADC. Don't get me wrong, i fucking hate vayne with a passion, but her main damage source comes from auto attacks, unlike other adcs who have better escapes, bursty ults, Q nukes etc. To compensate they give her the W steroid.
The reason why those passives like Ekko's have a timer is because they have abilities that can proc it in rapid speed. So they revolve around burst damage. I'd go with AUSTINODOOD's suggestion of reducing her auto attack range. That way when she's chasing you down you'll have more counterplay to escape, and if she wants to deal that obsurd %MaxHpTrueDamage she'll have to dive deeper into the fight and risk getting blown up. It'll make her more reliant on tumble to dance around.
Nerfing the AA range would also nerf her early game too, which she is supposed to be bad at anyways.