TO carry with vi its easy all you want to do is focus the one who is most fed.In a team fight start off my ult the carry to make them as useless as you can the go on to Q and e them till there dead.As vi your gonna be a bruiser so after you kill the carry go and get the rest of the back line.When you get the back line focusing you that gives time for your carry's to attack and there front line will have to back up to deal with you.You as vi can win team fight just by doing what i said as long as your a little bit tanky you should be fine and dont be to afraid to die.Most likely you ARE going to die but it will be worth if your team pulls through and that will allow them to win the game.I hope this helps you to play vi better let me know if you want to see some more build's