i reccomend on taking teleport so you'll be able to teleport to something next to your teammate/s and help with stun.
teleport is also useful in joining team fight in such a surprise that until they'll understand you're there they'll have a serious damage...
i also reccomend on taking exhaust as the 2nd summoner spell to avoid embarrassing situation when someone is about to run from you with 200hp or less...
in the start of each game, veigar's damage is usual... next to mid-game: you begin to get really strong with my build...
on late-game: if you've reached the 2nd archangel staff you're so powerful that if you have an ap champ against you you'll kill him on ulti+Q spell playing against veigar that reached late-game with this build is just unfair for the team who fights veigar...

GoodLuck with my build and Have FUN!