Whit this build your real AP is ~1200, because of archangel's staffs and rabadon's passives.

Veigar is very weak champion but whit good AP and high cdr it can kill all enemies whit 2 hits.
Veigar's ultimate is good if enemies have lot of AP players so he can kill them whit 1 ulti.

Use Event Horizon to stun enemies and when they got stunned use Dark Matter and Baleful Srike to make high damage.

Remember at farming to use Baleful Strike for stacking more AP. Every minion kill whit Baleful is 1AP and if you kill enemy champion it gives you 1-5 AP.

Veigar's abilities uses lot of mana so be sure you have mana to cast Event Horizon if you need to escape.

When you are taking build and you dont have space for 4th Archangel's sell your Deathfire. Adn if you want even more AP sell your boots and buy another Archangel's Staff.

I use this build and i can easily take enemies down but only bad side is that Veigar is slow and weak so play passive.

Hope you like and keep fun! :)

Sry for my bad english nor my first language. :/