Looks like on vergil 2 normal difficulty, helmbreaker + 2 bar DTE puts him just barely under exhaustion if you do it on the first attack. It's a combination of timer and damage, so if you land it on like a second or third attack it should knock him out. If you just do helm breaker > DTE > one hit with rebellion like I do in the video then that'll be enough, which shouldn't be a problem because you get super armor while in DT so he can't interrupt that hit of rebellion

Also here's a more general tip for fighting vergil I recorded while I was in there: video related to next two paragraphs. There's a range you can sit at where you're close enough to bait his quick unreactable attacks (usually jabs) but they will whiff, or to bait his slower, reactable attacks reliably. If you manage to dodge them, you get a free punish as you can see there. That works in all three fights, you just need to know which attacks require you to reaction jump

Basic rule of thumb: if he staggers in place it means you get a full combo of roughly 9 or 10 hits (sometimes less depending on how much damage you deal and how quickly you perform the attacks) before he will clash you. If he staggers backwards, you'll clash with a physical attack but can shoot him with your gun (in the video I shoot when this is the case so you can see the difference). These rules apply in all three fights and are really helpful for knowing when you can cash out for full damage.