Veigar is at his weakest in the early game. Try to abuse him to set him behind. Delaying his stacks will make him less effective in the later stages of the game.
Whenever Veigar uses his E, try to abuse the cool down as it’s his only defensive tool in the early game and once it’s down- he is rather vulnerable.
In order to get ganks or kill the Veigar, you need to bait out his E first. You can do this by constantly toggling in and out of range of him. Make sure you always bait it out before committing to the all-in.
Try and space accordingly to Veigar in team fights. Do not position too far forward as it will allow Veigar to catch you out with his E and start a team fight.
Locking Veigar down with CC will be the best way to deal with him in team fights. He is pretty squishy so if you’re able to get on to him and take him down at the start of the fight, your team will have an easier time winning it.
Veigar will use his Ultimate R to try and take down a low health/defenceless enemy. Buy items that will make it less likely that he can one-shot you with his Ultimate R. Standing as far back as possible will also reduce his chances of using it on you.
The longer the game goes, the stronger Veigar will be thanks to the unlimited stacking with his Q. End the game as quickly as you can.
Veigar is rather weak and vulnerable in the early game. You could try to abuse this to set him behind and miss out on stacks.
Once Veigar has his Ultimate R, his kill pressure increases. Keep in mind that he will deal a lot of extra damage at this time.