veigar is high burst champion. its very because of hes Q he'svery feared mid laner. most veigar payers start with tears of godness but in my opinion ROA is better. veigar is very squishy champion and needs tons of mana thats why buying ROA is better it gives you mana as well as health. and your Q gives the AP so you dont need the passive of seraph's.
staking is also very hard most veigar players throw their Qs too much and have no mana for other spells for example E and can't escape ganks. also dont forget to buy a vision ward. youre to squishy so your main goali before lvl 6 is to stay alive.

ask for gank when you get lvl 6 and try to get kill with your ult. recall and buy items. get back on lane and farm. last hit with Q dont push too much. remember mid game is where veigar is stonger than everyone else. at the time where nobody else has enough magic resist you are the worst nightmare for enemy squishyes and ap champs. try to stun as much enemyes as you can and burst them down.

at the moment when everybodys full build you're in danger so watch where you position yourself stay behind ally tank and wait. don't engage without your stun. use W on stunned enemyes or in the middle of a hard teamfght. try to kill AP ad AD carryes but never put yourself in danger if you can't get to ap or ad carry burst first enemy you stun. don't use flash offensively always save it for deffence. dont take your chance when enemy assasin gets on top of you use zhonya's imediatly.