If we're talking DmC Vergil vs Nero.....then Nero.
For starters DmC Vergil isn't fast that is teleportation.
Also by rules of OBD (Original Battlegrounds Domain) feats are measured by feats displayed during cutscenes or to a small extent gameplay (meaning their general moveset and how they work in combat rather than how players, a factor not included in OBD, can manipulate them.
Plus victor or who is stronger is determined by their feats and capabilities not their heritage or standing in their universe considering different universes have different rules.
Plus when addressing OBD fights one must measure the feats they displayed. Post Downfall Vergil has yet to display his "new found" power for all we know all he unlocked was the ability to control demons and the same can apply to Nero. So we're mainly using the feats they displayed until we see more of what they're capable of.
Now accessing who are superior one must access 4 factors. Speed, Durability, Power/Strength, Skill/Experience.
Speed: Speed is something the DMC franchise was heavy on versus DmC and characters like Dante could move at blinding speed and who did Nero go toe to toe with. Dante in DMC4 and by then Dante could be classified as Mass Hypersonic+ (Mass Hypersonic++ by DMC2) and the fact Nero not only was able to fight toe to toe with Dante, even if he lost the 2nd time and Dante was playing around that is still severely impressive, and was able to blitz him TWICE and was dodging close range bullets all with ONE arm is also more impressive. Making Nero a Supersonic at least maybe Supersonic+ in ODB terms. Plus with moves like Table Hopper and Shuffle Nero can quickly evade an attack and deliver a really quick and efficient counter attack. Plus Nero defeated opponents like Credo who move and attack at supersonic speeds making Credo Supersonic+ or Hypersonic so even if Nero's speed isn't impressive he can easily defeat (it looked like a curbstomp) guys faster than him.....that aren't Dante. Plus their was Sanctus who can teleport and Nero beat the sh*t out of him twice and the second time he had the Sword of Sparda so he had a significant power boost. DmC Vergil's movement is more teleportation than actual speed since it resembles Nightcrawler teleportation effect but in terms of speed in cutscenes and even gameplay DmC Dante has shown off far better speed and reflexives than Vergil although I do give Vergil credit that he is a bit faster than DmC Dante since he did blitz him ONCE in their fight and has shown to display quick movement during gameplay on a basic level. Speed is something that is very underwhelming in DmC and outside Vergil and the Dreamrunners but those can't compare to DMC4 Dante, Credo, and the Blitz (who could be faster than Dante) all opponents Nero could handle (except Dante). DmC Vergil is fast (the fastest in his series thus far) but I think in terms of speed though I wouldn't say Nero is better or faster but whatever speed DmC Vergil has Nero could easily handle it. But I give Vergil credit of the fact he can easily deflect bullets with his sword like the original Vergil and parry a lot of attacks.
Durability: This is a tough one. Both went through a similar situation of getting stabbed and getting knocked out and waking up to unlock some new power although the sword Nero got stabbed by was bigger. I say tie both were on the brink of death but came back stronger than ever although Nero got up much quicker plus whatever predicament Nero is he can DT out and summon that spectral demon to get him up quickly. Then there is vitality and stamina. I'm not really sure but Nero has shown great stamina he was able to fight Dante and win, face demons, fight and defeat Berial, face more demons, fight and defeat Bael, fight more demons and defeat an Angelo, fight and defeat Echinda, fight more demons, fight and defeat Credo, defeat an Alto Angelo, fought and defeated Agnus, fought Dante again, fought Sanctus and would win but got absorbed, and when he was freed you'd think he'll be exhausted or drained from being used to power up the Savior but damn he came in there and whooped everyone's ass and destroyed Sanctus and still had enough energy to finish off the Savior all back to back with almost no break. Not saying DmC Vergil could display a strong case of stamina and vitality but even DmC Dante did everything in moderation and day by day and not under a case of a couple hours like the plot of DMC4.
Power/Strength: Nero hands down. He even surpasses old Dante and old Vergil in this category. He was able to kill Bael just by slamming him hard enough and even punched back an entire an entire army of Bael/Dagons with a fist.....a SIMPLE PUNCH. Lets not forget he crushed the Saviors face and killed it while Dante could barely leave a dent. Plus Dante needed to Devil Trigger to soak in the damage of his punches so that implies or shows that Nero isn't to be f*cked with. Plus lets not forget that spirit that hover above him also attacks in conjunction so you're really getting slashed and punched like double the impact. I really don't need to cover Vergil in this one.
Skill/Experience: Nero may be a rookie well more a novice since he's been Devil Hunting for years already. He is more skilled and well trained at combat than the Nephilim brothers since he actually took lessons and training from a general for the Order of the Sword. Plus his swordsmenship is better than DmC Vergil. He may have recently got Yamato whereas DmC Vergil had it since childhood and in Downfall has shown to be quite skilled with it but he wield it quite skillfully and excellent I must add being able to dual wield it with Red Queen for a while and even when he isn't using Yamato the inner Devil is using it while Nero fights alongside the Devil so Vergil would be facing 2 people in one body at once. Plus due to the nature of Red Queen and its rev system being able to anticipate his next attack would be really difficult for Vergil making Nero's fighting style a bit more difficult to read granted for Vergil it may be a bit more straight forward but the demon and angel modes if Vergil uses them right could be used to his advantage. However, Nero was able to go toe to toe with Dante in a sword clash and can overpower him by using his devil arm (both cutscenes and gameplay) and he even defeated his teacher Credo quite easily too. Plus Nero is also skilled at hand to hand combat and various wrestling moves I mean those piledrivers and body slams don't come out of nowhere so if he is disabled or not using his sword he can just grab Vergil and piledrive him off a building (I kid) or punch the sh*t out of him. Nero is extremely well trained and well experienced. Plus Nero is just severely deadly and brutal.....no DmC Dante isn't brutal he is nice and soft compared to a dude who grabs his opponent slams them into the ground, proceeds to drag their body on the ground in a circle then throws them in the air, do spinning seismic body slam and the then stabs them with a bunch of swords before scrapping them away (poor Agnus) or better yet pick up a large demon by the face and deliver a series of extremely fast paced slashes and then slams him only to uppercut him sending him into the air and sending him flying with another punch (Berial) or better yet stab a demon in their gut and rev up his sword while inside their guts and not even yank it out he goes through the demon. Plus with how Neros's Red Queen and Blue Rose is designed and his sheer unorthodox combat style Nero is very efficient and precise on how he fights although DmC Vergil is more precise than his brother but still rather straight forward.
Although in terms of abilities Vergil has the Doppleganger and that gives him an edge but the way the Doppleganger functioned in Downfall isn't his TRUE Doppleganger considering the events of Downfall were in his head and that was stripped away version of his Doppleganger and his TRUE Doppleganger is how they functioned in his boss fight in the main campaign. That is one major advantage DmC Vergil has over Nero but other than that they're either equal in other stuff or Nero is better in most categories.
I'm sure it would be a tough fight but Nero might emerged victorious. I mean Vergil may be on his way to become a demon emperor but Nero has fought and defeated people who were practically Devil Kings (Berial and Sanctus Diabolitica/Savior) quite easily.