Viegar IS that 'lil buggar who bursts your health into nothingness. He is that mage who kills your mid, making him/her/it useless in team fights. I belive that Viegar is the mage killer that should be a couter-pick to any ap mid.

Your runes should focus on mana and ap like any other mage. I belive Mark of Insight, Seal of Replenishment, Glyph of Intellect, and Quint of Force fill this job perfectly

I follow the 20/0/10 build. Taking ap and magic pen. in the Offencive tree and taking the standerd mage Utility tree.

Opening with a Doran's Ring will in fact help you last hit with Baleful Strike.
Sorcerer's Shoes magic pen. is just gona help you last hit even better with Baleful Strike.
Getting your Archangel's Staff after you finished your your Sorcerer's Shoes will make a big impack on your farm, if you dont have this by lvl 9-11 your not playing Viegar right
Lich Bane is the PERFECT item to go with your Archangel's Staff
Are you a true ap caster with out your wizerd hat, ie, Rabadan's Deathcap?
Void Staff is just the icing on this little cake of evil

I start with Baleful Strike first, I max this first. At level two i pick up Event Horizen to prevent/enable ganks, I max this second. At llevel three I get Dark Matter and start my super farming, I max this last. At level six I get my ultimat, Primordial Burst, and let the fun (for you) begin, I, of cours, max this at level sixteen.

I tend to get Ignite and Telaport.
Becouse of Viegar's passive you wont need Clarity, but you can take it if you just want an extra strong early game, but it becomes useless late game.

Your a tiny master of evil and you should be acting like one, with this build you are that 'lil buggar that make the enemy team rage quit.