So last season I played veigar a lot. He was one of my mains in the support and mid role. I played I believe over 100 games with him in ranked total last season, and I finished mid plat last season. Just in case you were wondering. So I have tried a variety of builds on him and masteries and the best thing most successful strat that I did was one going mid veigar (because people would get their panties in a wad whenever I decided to support as him) taking teleport because the goal for me early was to just farm q as long as possible until I ran out of mana. Another thing I did and have decided to do is to take 20% CDR runes just fro maximum q stackage. Masteries was was first tree get deathfire touch. For landing your e I have a couple of tips fro that as well. So to start, using your e defensively is the best way to use it. People love to engage on veigar and the easiest way to stop that is to just sense when they are about to engage on you like with a jungler or just odd behavior and put the circle slightly behind you so that by the time you run backwards you will be inside of it and they cannot get to you with anything melee. For trying to use it aggressively don't try and put the edge of the circle right on top of them for this will cause them to be able to get away with little to no problem. Only try to hit them straight off with the edge of you are confident that 1. They are cced already and just to stack it. (YOU SHOULD ALWAYS NORMAL CAST IT THEN) 2. They are already getting away and you just kinda no harm no foul sort of thing.( To add to that even if they are clearly not going to be able to be hit by it (because of the delay) just throw it anyway you may freak them out and they might flash. Those are the only 2 times you should try and hit them with the edge straight up. As for more ways to use it you can trap them into it when your jungler comes and place your w at the back end of it so that they will either flash or have to come back to the front of it. Your e is also a very good way of taking away paths for the enemy like blocking a choke point like the path between the edge of the mid lane and the start of the dragon pit wall on blue side. Also do not be afraid to flash e for the good of the team. If you see the enemy team grouped up and your team is ready to teamfight, go ahead and flash e to make the enemy team either get stuck or waste flash before the fight even starts. And if you die doing that, usually you can get at least 2 people with your q w so your team can hopefully win the fight.
One last thing with the build itself it depends to be honest just where you are and how well you are doing but always include a 
, a 
, and a 
, somewhere in their. Also I would recommend never to build sorc shoes, really no need for it. Just some of the others depending on what you need that game hope that helps you and I can add you also if you want to know anything about veigar support or anything else
ign mostly conscious
if i want max damage. if i get any sort of a lead then I rush 
immediately because Veigar with a lead will usually keep it (don't bother if they have a Vi, though)