LF mid advice

If I'm Ziggs, what should I do when the other team has a fed Rengar top and a Vi jungle? No matter how far in the back I stand, Rengar can always blow me up instantly. If I quickly W away and throw my E down, he can still flash over it and E me. And that's assuming no one on the other team is nearby. In my last game although the only damage Vi built was elder lizard and triforce, she could still easily kill me 1v1 if I didn't have my summoners. She'd ult me, Q me before I did anything, then I'd W away and throw down my E. Then, she just kept chasing, Q'd me, I flashed away, she flashed towards me and killed me. And I had sorc shoes. (And both Vi and Rengar were in the game). So yeah, no matter how far in the back I played, Rengar and Vi could always kill me. I went for Zhonya's before Void staff because of it, but we lost before I could complete. I bought and placed 3 pink wards at different times, but Rengar always used his ult where my pink ward wasn't. What should I do in the future when the other team has a fed Rengar and a Vi? Because playing far back and cautiously really did not work.