Veigar needs a new passive! (This will probably be buried, but I'd love to get a Rioter's response)
Veigar's passive, 'Equilibrium', is basically a slightly weaker version of the 
and related items. While Equilibrium increases mana regen by 1% per each missing 1% of his mana, the chalice of harmony line restores 2% of the missing mana. While not identical, the differences are negligable, and actually slightly favor the passive as far as effectiveness, especially as mana pools increase (maths at the end of the post). However, I argue that Veigar really doesn't make good use of his passive. Champions like Xerath and Lissandra can get a free spell every 12-18 seconds, give or take, and in Xerath's case if he auto attacks a champion to proc his passive he might even get 2 free spells.
However, I don't think that Veigar should be made to be more similar to these champions. Xerath and Lissandra are about poking the enemy down and having excellent siege control. Veigar is closer to an assassin; one minute your squishes are there, the next they are not. He only has one AoE skill, and given the time delay it is very easily avoided unless the enemy is CC'd. I think it would be much more beneficial to give him a passive which augments this, in the way that Syndra's does (or even the passive on his Q). In fact, it would be better to remove Equilibrium's mana regen and replace it with a slightly stronger version of the passive on his Q. This way Veigar wouldn't feel bad about building items in the situations where he might not be able to survive rushing 
(which would be his ideal build path). It would also better tie him together thematically; a tiny master who is truly evil, who roams and tries to get picks and kills rather than one who is slightly confused and trying to lane a tad longer against someone who is far better at laning.
Here are the maths I promised:
Part I: Veigar's passive compared to other champions with mana regain passives/free spells
Even at lvl 18, where Veigar's base mana regen is essentially 4 per second (20 per 5), if he were to hang out at 1/2 mana and his passive increase his regen by 50%, he will only be getting an additional 2 mana each second. That's 40 seconds for the time it takes to get enough mana for a Q, a full minute for his E, and nearly 2 minutes for his ult.
-At lvl 18 Lissandra still gets a free spell every 18 seconds (regardless of mana cost) and can reduce the CD on this by impairing movement with her spells. -At lvl 18, Xerath gets 195 mana every 12 seconds, as long as he auto attacks something. This is enough for 2 casts of his E, 5 mana away from 2 casts of his ult, 25 mana away from 2 casts of his W, and this is all doubled if he should happen to auto attack an enemy champion.
Part II: Equilibrium vs Mana Font - Veigar's passive vs Chalice's unique passive
Chalice effectiveness > Equilibrium (just slightly)
lvl 1 Veigar: Base mana 350 (rounded) Base mana regen 6.0 per 5 (1.2/sec)
Equilibrium increased regen at - 1/2 mana: 3.0 (.6/sec) 0 mana: 6.0 (1.2/sec)
Chalice regen at - 1/2 mana: 3.5 mana (.7) 0 mana: 7 mana (1.4)
lvl 18 Veigar: Base mana 1277 (rounded 1300) Base mana regen 19.6 (rounded 20 per 5) or 4/sec
Equilibrium regen at - 1/2 mana: 2/sec 0 mana: 4/sec
Chalice regen at - 1/2 mana: 2.6/sec 0 mana: 5.2/sec
Now, these calculations show the additional mana regen given by each passive. While the regen granted by 'Mana Font' (the passive of Chalice) is marginally better than the 'Equilibrium' passive, it is still something which can be purchased by any champion for around 900 gold, with an additional 25 MR and more base mana regen as well. Given that this mana regen passive Veigar has doesn't help him enough to be truly useful (unlike Xerath's, Lissandras, or an actual Chalice/Athene's which has additional combat stats) and it doesn't fit his theme of a champion who tries to burst you down and walk away maniacally laughing, I think it would be far more beneficial to replace it with something which makes more sense for the champion.