Gain 10% CDR when you reach level 10. Excess CDR becomes AP or AD, Adaptive.
Unsealed Spellbook on Veigar grants you the safety of being able farm into the mid-game thanks to short Summoner Cooldowns. Flash allows you to avoid/setup ganks and your ability to take Barrier/Heal/Teleport keeps you safe.
Perfect Timing doubles down on your ability to stay safe in lane and scale into the mid to late game. After using your Stopwatch, don't be afraid to sell the broken watch for a bit of extra gold to hit an earlier Deathcap.
Minion Dematerializer is great for patching up Veigar's weak early game wave clear so that you can match roams with the enemy mid for jungle skirmishes.
Cosmic Insight further empowers your ability to stay safe in lane with short Summoner Cooldowns and the extra bit of CDR helps you to build your stacks even quicker.
Manaflow Band help you to sustain your mana during the laning phase so you have very little down time and can continuously build your stacks early.
Transcendence on Veigar allows you to hit 35% CDR in the mid-game with just Morellos and Cosmic Insight, allows you to build your AP stacks very quickly. By the late game you're able to hit 45% with either Banshee's or Zhonya's.