hi josh this is my viegar guide for you
enjoy :D

you want to start by pushing for a tear of the goddess, this will allow you to farm with your q for extra ap and keep you're mana up. late game you will want to build it into a seraph staff of a huge damage increase and a nice little shield.
you're primary item will be the deathfire grasp because it will allow you to 1 hit most ap carrys with your ult if you are doing well. the next item you pick is dependant on you're situation.
if you are dominating build towards death cap
if you need more mana build athene's or morello's
if you feel like you're cool downs are too long build lichbane

you want to start the game by just farming up you're q and trying to build you're gold uponce you reach level 4 and you have TOG try to stun your opponent and combo them with your w then q, this early it shouldnt be killing them but they wont be able to stay on lane for too long.
viegar works very well with blue, you should get your jungler help you with it as often as possable. after you get your deathfire grasp you should be able to kill you're lane opponent with 1 burst every time your ulti is up. remember to keep an eye on the other lanes for tele ganks, your stun has a great range and makes for easy kills.
when you reach team fights your primary role is to eliminate to opposing ap carry, play sneaky and wait till they are exposed or your team innitiates, as soon as you are clear deathfire the ap carry then hit them with q and r, unless they have built some hp, mr or you are behind you should ko them, continue to stun enemys and pick them off as you lead your team to victory