There is such thing as a powerspike with Veigar, and it is with items like
but your real thing you 100% want to focus and improve on is farm. Since you get AP for executing minions with your Q, you have a guaranteed source of power.
Every other laner that you'll fight against will have to get items to get any stronger, but you as veigar have the capability of having a consistent source of AP, often without even needing to back. So if you can focus on just stacking and stacking and stacking on top of never dying, you will no matter what become more powerful. Your items will obviously help with your power, but the amount of AP you can get from farming and harassing is promised, and consistent. You may not always be able to secure kills on your enemy laner. Some times they'll counter you, and some times they'll just be better than you. Some times they'll roam and get a kill putting you at a disadvantage but as long as the game is running you will increase in power, so the longer the game goes the bigger of an issue you should become.
Really with any champ farm is core, since its a consistent way to get money and power, but it hits home even more with Veigar.