Well, it's an off build so damage is always going to be the optimal way to go with him, but I don't see why this can't work. And as I mentioned, because he is going tank (to farm off his Q better), you can assume that he will be maxing CDR as a priority (FH+Visage does that nicely).
But as a tank with maxed CDR, he would be going a more supportish than carry Veigar with more frequent use of his E and provide his damage in a stream of 3 second Qs, and 6 second Ws, compared to a single 1 shot burst. His ult mechanics also ensure that he will always be nuking the enemy AP carry regardless of build, and if they don't got one, you'd win by rushing Thornmail anyway.
I am aware that 2 games is nothing, which is why my next project after ap Malphite will be tank Veigar (going to master him with only playing tank). I'm sure the bigger picture will surface then, which might only mean that it's good at my (low) elo or maybe not viable at all. Who knows?
Edit: I found that diving into the middle of the enemy team (stupid thing to do if you're a squishy) and putting an E on yourself is a good way to land 5 man stuns because everyone's reflex is to dive on you. Happened in the Blitz game whenever I got pulled.