Veigar depends from mana (only on Early game). You are one of the best nukers in game so you need Ability Power. For me these are the 2 basic things for Veigar. These runes are full of power for character like Veigar.

I'm playing 9-0-21 because this is mostly the best way for casters. First get Offense masteries and then Utility. Spell penetration need you. It will help you a lot in team fight or in each fight.

Veigar depends a lot from items. For that you must farm in Early game and to get items. Use the items up if the enemies don't focus on you. If the enemies use magics a lot get Banshee's Vale. This item is more powerful than you think. It helps you so much. Get Abyssal Scepter, too. Or if your enemies make basicly physical damage get Zhonya's Hourglass for armor.

My choice for Summoner Spells are Flash and Ghost. I think this spells are good for nuker like Veigar. I chose Ghost because Veigar is annoying slow. With Ghost he can run away from enemies and get near to them. Flash will help you a lot. It's a powerful spell.

Veigar is made of paper so in Team fights stay behind your teammates and let them to tank. When you push and enemies are hiding care because they will try to focus and kill you. Your character is a ganger so make how you can more and more gangs and help your teammates with lane control. Don't let enemies to get more powerful than you because then you can only just quit! Veigar makes a lot of magic damage and don't afraid to use your magics on enemies because there isn't item like Thornmail for magics. You can ignore enemies' Magic Resistance with Void Staff. Try to don't let enemies make damage on your team.. You have mass stun USE IT!

Veigar is a overpower character (late game character) and for that u have to play more closely. In the first lvls (1-3) u need see that are your enemy/s are playing aggresive or defencive. You need faming minions with your first abbility to get up Ability Power. Veigar is a nuker so try to denied your enemies to creeping. Farm everytime when you can!