M usually, but you have to adjust depending on what the opponent does. Might have to use H version. Just be mindful of the fact that your short ranged game is severely gimped when charging with L.
Oh man. DT+ missiles. I could write for years. Best air dash in the game, a double jump, increased hitboxes and chip damage, but if that weren’t enough, the real reason DT is just way too good…
The ability to whiff cancel vergil’s normals. This is totally unfair. His st. H already negates projectiles, so you could literally just whiff st. H xx judgement orb (orb’s hitbox in dt is silly) all day and outzone a massive majority of the cast. Taking it to the next level, you can whiff cancel your stingers as well. This is important because apart from the gargantuan range/lockdown, it allows you to adjust your spacing, empowering you to zone very aggressively. It’s not something I would do often, but it’s worth mentioning that the startup of judgement cut and rapid slash are very similar. That means that every time the opponent holds forward for a rapid slash, they get bonked by a judgement cut (you can get some neat confirms off this, especially when you throw missiles into the mix). Likewise, when they start blocking orbs, it opens up a prime opportunity for a practically guaranteed rapid slash. Begin by chipping them to death, get a feel for their skill level, and adjust accordingly.
If you know how to glitch, DT is just that much better, as it has the added benefit that if you do manage to whiff your stinger/st. H/whatever, instead of getting punished, you cancel immediately into round trip and get a free mixup, or just opt to maximize your chip.
Also, the air dash in DT is downright oppressive, and I don’t think I need to note that double jumps, especially shallow ones (vergil’s is almost strider-esque), make for some AMAZING mindgames, and really expand your options for opening an opponent up. If you’re one of those people who say “ermagerd, vergil why u no mobility on incoming?”, DT is also a good answer for you.
Finally, learning to use st. H xx DT and judgement cut xx DT open up some incredible option selects, and even relatively easy guardbreaks from time to time. So keep playing with it. Swords and blistering swords mixups are usually what I go to if I’m trying to open someone up. If I want to chip my opponent out or increase my mobility, or just outright survive, DT is undoubtedly the way to go. It’s pretty good for recovering red life quickly too.
At any rate, I’ll leave you with a good example of how orbs and DT can open an opponent up.
Opponent blocks your cr. LM, and expects a rapid slash after the M. However, you did cr. LM and instead cancelled it into judgement cut, usually a L or M, and cancelled that into DT. The opponent holds forward for rapid slash and gets opened up by the orb, and by pre-empetively cancelling into DT, you can dash up for an easy st. LMH rising sun confirm. This is just one example, play around with it and see what you can come up with!