Who knows. Wlad was much, much quicker than Joyce. Zhang got very quick and heavy hands but same time his general speed is average. He is lazy sometimes and got average stamina too. U can see Filip Hrgovic who is much quicker than Joyce looks much better. Wlad in prime is not soo though as Filip but fighting much smarter and take less punishment for sure. I think he was little quicker than Filip (to this day because I think Filip still got time to improve) and got better footwork. Probably Wlad will be winning on points and got power to make Zhang pay attention to defence. All succes in that fight for Wlad will be not take very clean shots. Easy to say but life is life.
We can judge Wlad only based on his fights against Fury and AJ because Zhang is closer to them than to Povetkin or Pulev. Heavyweight was in a terrible state during the Klitschko reign. Most of the top names during this time wouldn't stand a chance today.
All I can say for sure is that Wlad would be extremely cautious against Zhang, as would be Lewis. I think both of them have enough skills/punch anticipation to avoid getting hit early, and enough power to take Zhang's respect. Both would likely box very cautiously in the first half of the fight to clearly win the second one and probably stop Zhang. But they would need to be at their very best to do it
Wlad would peck away from the outside for half the fight trying to get used to Zhang's style and speed, as he usually did. Second half his superior fitness would tell and I think he'd take enough rounds to win a comfortable if boring decision.
If Wladimir gets caught early though it could be curtains, but i think a favour Wladimir in the 2nd half of the fight aswell.
Wlad became really effective against southpaws under Steward. I think it may resemble the Ibragimov fight somewhat with Wlad being cautious and using his lead hand to knock down Zhang's right so he can land his own right hand.
The confident, aggressive Wlad that pieced up the remains of Ray Mercer would batter and probably stop Zang. That Wlad combined speed/athleticism, offensive creativity, and genuine finishing power. He really was a pleasure to watch in the early years.
There was a reason Vitali took out Corrie Sanders for Wlad. Any version of Wlad would have would have trouble against a hard hitting southpaw with size and the ability to counterpunch from range.
Before we see Zhang against high quality operator like Fury or Usyk, all those Lewis-Zhang, Wladimir-Zhang threads are like second wave of Tuamania. Zhilei already has a legit loss on his record in Hrgovic, and ways to beat him were already displayed.