Why would you turn on Devil Trigger that early during XF? The whole point of XF with Vergil is to use XF to build meter for Swords and more DT when the XF runs out.
The video is really just an example of combos during both states, but If I already had 3+ bars I see no reason to not remain in Devil Trigger throughout XF, he's essentially a boss character in that state and can easily run though an entire team of high health characters before XF has run out.
Yes, he can meterless ToD off of optimal hit confirms in XF3, but they are both somewhat finicky and time consuming in comparison when he could just activate Devil Trigger, kill in 3 seconds, and get to run his Rapid Slash/Judgement Cut/Throw mixups on the next character and usually kill again with the same Devil Trigger. The general idea is to win the game in under 20 seconds. When not dealing with 3 high health characters, you're probably going to win in much less than that. It's really quite broken.
Also, I said in the video description how XF1 + DT is faster than just XF3, but I didn't mention that everything except the double Rapid Slash combo and the late combo self otg loop works even in XF1. Thus, for second slot Vergil teams, he can have access to 15 seconds of being nearly as absurd as in XF3, but with an assist. Considering that this should only be done off of an initial hit confirm, it's pretty easy to murder two characters in record time or even flat out win the game before XF2 ends.
Formal Introduction:
Hi, I'm a scrub from the NJ area who gets bodied one game away from playing on stream at Big Two on a weekly basis. I made that Dark Vergil video. I also made the video with Dante building 3.25ish bars that everyone went nuts over. I've been lurking on here ever since I was waiting for my account to get activated, but still didn't post until now for whatever reason. I've posted a lot of stuff in my character's forums on SRK, someone might recognize me.
Also, about 4 weeks ago I did the unthinkable and dropped Zero. Dropping Zero/Dante/Magneto for Dante/Vergil/Strider isn't really falling that far off though.