Assuming you are using Vergil on point the most important thing on that team is to know Vergil's normals very well. Knowing Vergil's range tremendously improves Vergil's mixup game since one s.H call Mags cancel into Trick L/M can lead to death for that character.
Against characters who lack mobility options don't be afraid to zone them out with judgment cuts and round trip along with magneto. If they super jump, dash under and repeat.
Just some things I do with the team. There's a lot more, but it wouldn't be fun if you don't find out for yourself ;)
I'm still learning Magneto but I'd recommend watching ALOT of Fchamp's matches or Rayray's matches. Just watch how they move and see how they convert random stray hits into 800k+ combos even from super jump height.
The Magneto combo I've had the most success with online is whatever(usually lmhs) H > airdash > M flight mode L H then alternate dash H dash H dash H and then addf H and then go into hyper grav loop. I would not recommend the magnetic blast loops if you play online, though if you're playing offline go for it.
Also when doing the Hyper grav loop use L + M to airdash down because if you use H in the airdash Mangeto's JH does not come out on time. --- http://steamcommunity.com/id/toilethumor psn : toilethumorr