You'll need to give some particular areas where you struggle. Looking through Match History, your itemisation isn't very good. Due to Veigar's nature it's easy to believe you need to build a lot of Mana or Mana Regen, but that's not really true. Mana Potions are sufficient, Morellonomicon OR Athene's Grail supplement it later on. You want Sorc Boots, Zhonya's and Void Staff before you look toward other items. Why Void Staff? Because with Sorc Boots and Void Staff and Runes/Masteries, you should have 23 / 39% magic pen. Against a 55 M. Resist champion, that's ~11 M. Resist left over if I do my maths right, and that'll boost your damage against them tremendously.
"Hold on, Ramuhdo, Veigar has great AP ratios, Deathcap's better!", you might say. Deathcap only boosts your AP ratio part of the damage, Void Staff gives a % boost to both the base AND your scaling damage. Also, against 40 MR it's 1.4 MR left. Against 30 MR (mid champs who don't believe in MR), you do True Damage. Trust me, you like that.
If you max your Q first and your E second, you'll have an easier time dominating. E is a good zoning/CC tool, and most of W's damage'll come from your AP ratio. Higher level E lets you actually land your W. If allies can keep opponents in place (or if they for some reason don't dodge), it's fine to max W after taking E to level 2 or 3.
Stay on the outskirts of the fight, poking in with Q's and E's to try and generate picks, then blast the enemy squishy with justice. If an engage happens, or a fight is underway, either use E around enemy carries to limit their mobility/stun them, or put it around YOUR OWN CARRIES to protect them from people trying to go in on them. Veigar's awesome like that.