As veigar, Veigar’s level 6 is stronger than yours. With this in mind, play safer at level 6 and keep your distance so you have more time to react to his cage + all-in combo.

Stand away from the minion wave so he cannot push and poke you down simultaneously.

Play around his E cooldown. If you dodge it, abuse the long cooldown to gain a significant health advantage.

Try and force him to over extend for farm by keeping the minion wave closer to your side of the map. You, your Support and your Jungler can then run him down the long lane. As veigar, With this in mind, play safer at level 6 and keep your distance so that you have more time to react to its cage + combo all-in-one. Stay away from the minion wave so it can't push you and fall at the same time. Play around its cooldown E. If you keep it away, abuse the long cooldown to gain an important benefit on health. Try and force it to extend for the farm by keeping the minion wave closer to your side of the map. You, your Support and your Jungler can then get it down on the long track.