Use Event Horizon to increase your chances of landing Dark Matter.
Veigar is extremely Mana-hungry early. Try buying Mana Regen items to increase the effectiveness of your passive and allow you to cast more Baleful Strikes.
Veigar is very fragile. It is valuable to select at least one summoner spell that can be used defensively.
Dark Matter deals very high damage, but it can be avoided. Pay attention to the sound and visual indicator to be aware of when and where the spell will land.
Event Horizon only stuns units on the edge. If you're inside the spell, you can still move and attack.
Veigar's ultimate deals damage based on your ability power as well as his own.