Headcanon is a go!
1) Vergil, as a child, always felt that Eva preferred Dante over him. Dante was a happy-go-lucky child, who would be quick to cry but even quicker to laugh-- Vergil was childish, too, but he was much calmer than his twin, and so felt that his mother didn't love him as much because he was unable to be as open as Dante. When it came time to play, Vergil knew that Dante would go and rush towards Eva first: this started Vergil's relatively quick maturity, because he felt like he was unable to be as cheerful and immature (in a good way). From a young age, he would often scold Dante for 'naughty' behavior, as if he were much older than his twin.
"Dante, you can't do that."
"Dante, you shouldn't say that."
It was all well-meant, but whenever Eva would laugh and tell Vergil that he didn't have to be so proper, that he could be a child too, he would feel like he made a mistake. However, Vergil's maturity seemed to suit Sparda fine, and that made Vergil admire his father even further and made him want to emulate him.
2) That's not to say that he didn't get into his fair share of troubles. When Dante would suggest something to him in just the right way, he could be coerced into doing mischief. Those were often the most successful cases of managed mischief, but also the most serious.
But Vergil never once would claim to his parents that it was "all Dante's fault." He took the blame equally, even though he would go and scream and fight over it with Dante later.
3) In that vein, even fights between the two as children were fairly intense, and would go to the point where both of them would be yelling and hitting each other quite violently. These fights would either stop with Eva breaking it up, Sparda butting in, or either of them starting to cry.
Fights usually ended most quickly when either Sparda came in or Vergil started to cry.
4) Vergil liked reading from a relatively young age.
5) Sometimes, when nobody was looking, young Vergil would sneak into his parent's room to look at Yamato. He'd often tell his father that that was the sword that he wanted.
Eventually, he got his wish.
6) As a child, Vergil was quite protective of his twin. As an adult, he's protective of his twin in all the wrong ways-- as in, he won't allow Dante to be defeated by anyone but him. The reason why Vergil sees defeat by Dante such a personal disgrace is because he always felt that he was the stronger one, that he was the one responsible for 'protecting' Dante-- this skewed vision of 'Dante is weaker' makes him angry that Dante doesn't pursue power in the same way that he does. Since Dante is weaker, and since Dante no longer has Vergil to 'protect' him, his brother should be seeking more strength, in Vergil's mind. He finds Dante's reluctance to take on Sparda's legacy as a sign of that inherent weakness, that human trait that Vergil's come to despise. He finds it foolish and immature: he thinks that Dante will be killed, all for the wrong reasons. Therefore, if it comes down to it, he'll save Dante from death by another if he has to.
He'd never admit to that.
...He'd never admit to any of that.
7) Vergil has a strong appreciation for ancient Japanese culture, his research mostly stemming from his initial curiosity about Yamato and katanas. He has a somewhat skewed vision of Japan, as a place with honorable warriors and age-old, valued traditional customs. He's also proficient at using chopsticks.
8) He'd only trusted Arkham to provide him information and to do the grunt work: he'd always sensed that Arkham had ulterior motives, and was thus planning to dispose of him from the very start. However, Vergil's pride and his general underestimation of people who aren't him led to his initial defeat.
9) Vergil cannot hold his drink. At all. Despite his demonic...tolerance, or whatever it is. Feel free to exploit this.
10) Vergil would profess to hate pizza, but he doesn't. His favorite is pizza margherita.
11) His favorite operas are 'Don Giovanni', 'Faust', and 'Der Freishutz".
12) He has the tendency to take a lot of things personally, and in that same vein, he can be pretty selfish. Thus the "the decimation of my family was my fault, so now I'll destroy the boundaries between humans and demons to get father's power" idea.
13) Vergil doesn't hate Dante. He thinks Dante is weak. He thinks Dante is an idiot. He thinks that Dante's done disgraceful things. Dante is everything that Vergil would hate, if any of Dante's traits were his. But since they're different people, despite being twins, Vergil can only resent his brother's decisions and his personality, not Dante himself. It's a deep-rooted anger and disappointment, but not hate.
14) There are three triggers that can set Vergil off: personal disgrace, mention of his past, and his brother. Even as Nelo Angelo, these things were triggers. That says a lot.
15) He can't stomach sweets. Period.
16) Dante may love the ladies, Nero might be shy with them, but Vergil? Not interested in ass or titties. HE'S GOT MORE IMPORTANT THINGS TO DO OKAY
17) The voices in Vergil's head, the ones that tell him that he has to become stronger, the one that tells him to kill and kill and kill and kill? He's begun to interpret it as Sparda's will. He's strong enough not to give in to the voices, but Vergil's started to interpret that demonic nagging for more power as his father's words to him.
He knows it's not true.
He pretends not to.
18) He's proficient in most European languages, though only for literary purposes-- mostly because he learned them through classical literature, and most of his vocabulary tends to be stuffy and archaic.
19) The demonic whispers that Vergil hears in his mind turn into a burst of choirs when his adrenaline really goes up. If Dante and Nero hear industrial or metal when they're riled, it's the complete opposite, for Vergil. For him, it's a sweet, plaintive, yet distinctly insane form of choir music.
20) His true weapon will always be Yamato, but he must admit that Beowulf is fun to use.
21) He can't stand having his bangs be down for a prolonged period of time. He finds it irritating, not to mention too reminiscent of his brother. It's a bit of a habit of his to brush his hand through his hair.
22) His own personal Hell would be to be forced to sit.
And watch.
The night his mother was killed.
Over and over.