How much will the veigar nerfs affect veigar?

It's 10-30 damage off his Q and 20 damage flat off his W. Max of 50 damage lost. It's... really not much. It'll annoy a lot of Veigar users since the Q has been nerfed in base damage, but teamfighting-wise, it's really nothing too bad.

Will hurt his early farming a tad. But late game is basically unaffected.

So far the nerfs are W damage reduced by 20 at all ranks and Q damage reduced by 10/15/20/25/30.

Let's say Veigar gets 8 assists per game (average according to champion.gg is 7.67 - close enough!). The earlier Q buff/change means he gains anywhere from 8 to 40 AP from his Q passive from champion assists. That's 4.8-28 damage buff on Q, 8-40 damage buff on W, and 8-40 damage buff on R. This means his W still got a net buff if he has at least 4 rank 5 Q assists, and his Q got a mild net nerf that requires one more assist over the average or ~half a minion wave Q farmed to go even.

So overall, not much. He'll still have his lategame power roughly the same or better, but his earlygame and midgame are weaker and rely on his AP stacking a bit more.

They're targeting his early farming (like level 1 q's) which I believe will have a unusually noticeable effect. Q's damage early on is really pitiful and chances are after the damage is changed to 70 at level 1 you have to time last hits with it a lot better. But his late game is basically the same

They're nerfing around the problem area, which seems to be a relatively constant aspect in Riot. I doubt this nerf will do anything in the long run.