Thanks, if you see anything that should be changed, let me know

Good build, works perfectly

I was not aware that baleful strike gave you ap when killing minions I only thought it was for champions. As for event horizon I don't recommend using either Dark Matter or Event Horizon until the later levels due to the heavy mana use, I would only recommend using them at around level 7-8, and by that point you would have both anyway I only picked event horizon first to help you get out of any dangerous situations if it comes to that.

For the Deathfire grasp/archangels staff I have found this setup to work the best for me and it flows pretty nicely.

for insight, I am typically soloing the middle and so insight doesn't really help much, and when I am taking another lane, it is usually with a tryndamere so again it doesn't help at all.

This is just what I use and it seems to work well for me, do with it what you will but this is what I found works the best.

Veigar doesn't Insta kill most tankers, even when farmed A LOT with baleful strike.

Personally, I'll take Insight(Mastery for Clarity) to give more mana to teammates over Persevereance, which does pathetic healing rate, which you don't need late game when you have good mana regen. I'll suggest either Deathfire Grasp (for tanker teams) OR Archangel's staff, instead of both THIS early, so you can farm Rabbadon earlier. By going AS and Rabbadon's, by mid-late game phase, I have over 400 AP (155 from rab, 45 from AS, 3% of 3000 meaning extra 90, making the total 290, + baleful strike last hits when your lane partner kses like a G6 (Roughly 60+), making a total of 350+ AP and a bonus 30% which gives already nearly 90 AP w/o the baleful strike bonus. To me, there is no point adding Event horizon to lvl 2 before maxing dark matter, the extra stun time is 1 sec from lvl 1 to lvl 5! Sides that, your guide is undetailed and overall not good to description, etc baleful strike increases your AP by 1 per minion kill with it and 5 AP per champion kill (any spell) after you have added at least lvl 1 baleful strike. As you mentioned, Veigar IS SQUISHY. Anyone such as Evelynn can stroll past and 1 hit KO veigar, or at least stun and kill him before anyone reacts. No escape spell for a rusher type like Jax or Xin Zhao? Its really risky and very hard to play well, unless you are expected to die over 10 times in the game.