This is no offtank. This is some AP, some HP, no real damage, no real resists... yea, you have a GA, but you are going to die instantly anyways. No matter that you have tons of hp, without resists, any ad will be able to simply tear through you...

Your item build should be soemthing like Merc Treads, Void Staff, Rabadon, Ans possibly warmog, force of nature and frozen heart. Basically just strong defense items, since veigar is such a squish without items.

About shoes - You are NOT an ap carry. you said it yourself, you want to build an offtank..
Rabadon and voidstaff - you cant get more damage out of two items, and you need at least some damage, or you will not be noticable at all.
Warmog+FoN+thornmail, randuin or frozen heart, depending on enemy team... if you wanna survive, you need strong resists...

Honestly, i dont think a veigar tank can work, but if you make it work, be sure to tell people that he is basically nasus 2.0, just Q,Q,Q... if you don't farm your Q, your damage will suck either way... (and if your damage sucks, the enemy can ignore you once you land(hopefully) your cage)

1st item Deathfire Grasp ? 2nd Exhaust for Veig..? ur nub srsly.. (lol) 3rd you start too late your deff items. 4th if you wanna survive Will of the Ancients and Rylai's Crystal Scepter and theres MUCH more but i dont have time... L2P.