Veigar is weak early game, but builds up over time. So, I've chosen items and runes that help him reach that mid-late game. Your goal is to survive the laning phase and assist your Marksman in getting gold by auto-attacking cannon and melee minions. Be careful not to over-extend.

Think of your wall cage as a defensive tool. If you use your cage wall to engage and it misses the enemies are free to hop right on top of you. Let things play out a little, don't panic! Your cage wall takes getting used to! :)

Guardian works because it scales off 25% of Veigar's AP. It also is a good uh-oh in the laning phase. Try to stay close to your ADC, the movement speed it gives will be helpful in running away or chasing.

I max Dark Matter first because it does more damage than Baleful Strike; it scales off 100% of the ability power from items and your passive unlike Baleful Strikes 60% AP ratio. Dark Matter is very rewarding and can do big damage even if there is a little more risk involved. You usually don't want to be just dropping bombs everywhere early game, just use it to keep the enemies in check.

Baleful Strike is relegated to being a tool that you use to poke enemies with to build up your passive.

There is a Delay in Dark Matter (and Event Horizon) but that's part of why I enjoy it so much.