I think you underestimate the value of having Clarity early game. Against a passive lane opponent not having Clarity is fine because you can conserve you mana by lightly harassing and last hitting. However without it against an aggressive lane opponent you may have to return to base to often because you may not be able to return the damage while still conserving mana. With Clarity I can be the more aggressive laner, forcing my opponent back instead of turret hugging with low health and no mana. Ideally I would have Blue Buff but I play solo queue mostly and cannot have it early game when I would need it.

As far as my masteries go, I prefer the extra experience and cooldown reduction over the extra damage. The 5 and 6% increases seem too small to to take over my masteries in my opinion.

My item build does leave me quite squishy but as Veigar is an ability power carry, trying to build him at all tanky would be very ineffective(the spell vamp is for farming health so you don't need to return to base). With superior positioning you can stay out of range of most champions, go in to burst down an opponent, retreat to a safe distance, then do it again. The range on Abyssal Sceper's passive is 1000 while the furthest range on one of Veigar's spell's (Dark Matter) is 900 so you are able to keep your distance while providing the debuff. This makes Abyssal Scepter more affective against enemies with around 50 magic resist while also giving some magic resist.

Clarity is not a good summoner spell. You should be able to ration out your mana effectively without it, and the extra mana does not scale well into late game. You're better off just asking for (or grabbing) blue buff.

You're missing out on a few very powerful masteries in the offense tree, such as Archmage and Executioner , which would boost your damage output. There's nothing on Utility that really compares to them.

Your item build has two main issues. First, it leaves you extremely squishy, as only a single item provides any HP, armor, or MR (spellvamp doesn't make you less squishy). Second, you'd be far better off switching the Abyssal Mask for a Void Staff, in terms of damage dealt. Abyssal is only useful against enemies with low amounts of MR, and if you're doing well it's fairly obvious that everyone will pick up some MR to counter you. Abyssal is also mostly useful on champions that are going to be close enough to the enemy team to get most or all of them inside of its aura (such as Morgana for using her ultimate); Veigar is completely ranged, and wants to stay on the outside of a teamfight, so you won't get the whole enemy team with the Abyssal aura, lowering it's effectiveness. You should take Void Staff over Abyssal in almost all situations with Veigar.

These issues, combined with the general lackluster-ness of your guide (you don't explain much about how to actually do anything, other than emphasizing farming with Q), put me over the edge to -1.