It’s a frustrating matchup. Most Veigar’s I’ve faced start flask making early harass mostly pointless. Try to make him waste time and cs harassing you while you farm up. Once you’re both 6 it’s a game of avoidance and making someone miss a skill. Whoever stuns first wins the fight.
Veigar has a significant advantage due to his stun being instant but if you’re good at teamfighting with viktor you can usually scale up and remain relevant regardless of how the lane matchup goes.