The Yak of any kind is out of his depth at those altitudes. Against a Yak-The higher it goes, the better the pony gets. You have speed and therefore vertical on him. The pony also has a great instantaneous turn and a much longer range. Stretch him out on his fuel then drive him crazy when he wants to go home. It's what Jazzi would do.
Keep the speed up and keep dropping on his head. If he keeps doing that roll to force you out front compensate for it and try and shoot where you think he will be. Rinse and repeat. The Yak is a small and nimble target. Very hard to get hits on.
You do have speed but not a lot, Vudu was cruising at 350 at the start of the film. The 51's speed at 20k is about 390. I think the 51 would zoom climb better if you wanted to try to take him up, make him stall, and hammerhead on him. Dazy controlled the fight for most of the film. I think Dazy could've been a bit more aggressive but how much more before he got sloppy and into a turn fight with the yak? I think It's a slippery slope. So what do you do? You BnZ him, and it goes back to my list of things that may happen.
1. You will get a shot and make it happen.
2. He will turn to HO you.
3. You will get sloppy with your E and he will teach you a lesson.
4. The Yak will dive out and run home.
Hopefully you can try and predict what he's going to do and get the shot on him, like you said. If you try to take him up but he's to smart to fall for it what then? You're stuck BnZing him? What if you keep trying and just can't make it happen like Dazy in the video? You go right back to my list. He's going to HO, or run home, or you're going to get sloppy and die.
I only skimmed the video but it seemed like the pony had an initial E advantage that was not pressed and therefore was neutralized.
If you are BnZ or E-fighting someone, the objective is to reduce their energy to the point where they can no longer evade your shots and the longer you extend, the more time you give them to build E. While taking a half-hearted pass and then extending for several thousand feet may keep you alive, it also helps to keep them alive. The pony basically did nothing to help his own cause here.
I agree with you, He should've kept pressing and draining Vudu's E. I think it was smart to extend but he did extend to far and allow Vudu to get his E back. It's a tough call trying to figure out E states, when do you press, when to extend, ect.
Just fly off map to avoid getting shot
You know, It's funny. People go off the map every day and nobody says a thing about it. We go off the map because we're being vulched on the runway for our mega mission and you won't stop crying about it. Why don't you go ahead and pound sand.

You too.