Just a few problems with this build. First, you want the Summoner Spell "Clarity," not "Clairvoyance." Second, Marks (red) should be Magic Pen, not Ability Power. That's what Glyphs (blues) are for, and you made the right decision in going with per-level instead of falling into the flat trap. Seals (yellow) should be either Mana or Mana Regen, and Quintessences should be AP, not Cooldown Reduction; getting the right items will do that for you in a heartbeat. Third, items: You want the Sorcerer's Shoes because if you don't, enemies will start stacking Magic Resist and you'll be up a creek. The Banshee's Veil is more of a defensive item, and Veigar is meant to be flat-out scary magic damage. Get either Deathfire Grasp or Abyssal Scepter instead. Shurelya's Reverie is another good Cooldown-Reduction item. Fourth, Veigar isn't a bad midder, per se, but he's better off laning with a tanky-person. It's just safer that way. Fifth, your skill-order is odd; you really shouldn't focus Event Horizon until after you've maxed Dark Matter. Two levels at level 13 is totally acceptable, and the reason is that a few extra levels in Event Horizon don't make a difference if Dark Matter and Baleful Strike won't finish the job. Sixth, masteries should be spec-ed basically the opposite of what you have. Veigar is gonna die; he's an early target in team fights if he knows what he's doing, and he's too squishy to stop it, so you're better off with points in Good Hands. Also, an improved mana pool is more important than 3 mana regen per 5 or 30% longer creep buffs; take out those five points and put them into Good Hands and Expanded Mind. Seventh, your icons are just a touch too small. Set them to size 24 or bigger, and you'll be fine. Eighth... Just run this through a word processor and watch the red and green lines show up. It's kind of atrocious. Use punctuation, capitalization, and spelling properly--it makes you look a lot smarter and more professional.
I apologize for sounding like one of those people who has nothing nice to say. I applaud your effort in putting together such a complete build, but make it more correct.