If I control a bf with Lonely Poro and I hide a Zhonyas there, my opponent attacks with Vex and kills my Poro, can I react to the Deathknell with Zhonyas or my Zhonyas would die before because I don't control units at the battlefield?

Yes, you can flip Zhonya's in response to the Deathknell trigger — but it won't save the Poro, and the Zhonya's itself is fine.

Here's why each part works:

1. You can react to the Deathknell trigger. When Lonely Poro dies, its Deathknell goes on the chain, entering a Closed State. Your hidden Zhonya's gains Reaction (Rule 811), so you can flip it in response to that trigger on the chain. You have a window.

2. It won't save the Lonely Poro. Zhonya's is a replacement effect ("if a friendly unit would die") — it needs to be active before the death occurs to replace it. The Poro's death already happened by the time its Deathknell trigger hits the chain. Flipping Zhonya's now gets it on the board but can't retroactively undo the kill (FAQ #10998).

3. Your Zhonya's won't "die" from losing the battlefield. While a Combat is ongoing at that battlefield, you don't lose control of it — Rule 187.4.b says control can't change during Combat, and Rule 187.4.c specifically exempts battlefields with an ongoing Combat. And even if you did lose control (e.g. after combat ends), gear isn't killed by losing the battlefield — it stays where it is as your permanent, independent of who controls the location. Unattached gear at a battlefield gets recalled in cleanup (Rule 149.3), but that's a separate process, not a kill.