The play style of this build is early to be stingy with your spells other than Q, your mana costs will be to high to spam spells till around level 4 or 5 and your cage is what makes you hard to gank, as soon as you get your first item its time to switch from passive to aggressive punishing your enemy with e w q when he comes in for cs

You need to change your build order depending on the lane opponents,an example would be don't rush frozen heart against an Ap mid laner maybe the Rod of Ages would be better or if you plan to passively play mid and be aggressive roaming bot rushing frozen heart to better match up against bot lane or jungler would be a plausible option "TLDR plan your items with how you want to play the match" if your matchup is bad playing mid passively and roaming bot/top can win the game easily while teleport keeps you from getting punished to hard.

Its best not to engage with your cage its true that the chance for a pick is nice but you just lost your only pressure spell other than r q oneshot the best way to use your cage is to wait for your team to engage then pop it or wait for your enemy to engage then use it, of course if your ahead no one can punish you and with this build you can even frontline as a mage for a bit just don't be the first one to enage or you will become the priority target

If you are being out farmed more than out farming your enemy, veigar isn't for you there are champs that don't get hit as hard for not farming such as cassiopeia or syndra but its important to remember without farm no matter how skilled you are at league of legends you will be much more likely to get rekt in lane than to do the opposite, farming wins games and should always be a skill you should practice and be self conscious about

The best combo with veigar to me is flash teleport but as I said near the top planning ahead makes a big difference even in summoner spells taking flash should always be guaranteed but teleport can be replaced though it helps keep your lane pressure something like Barrier or Cleanse heck even Exhaust keep an eye on your match ups and take what you think you'll need

Telling you where to ward is difficult in text but it wouldn't hurt to google a warding guide it can help more than you realize.

I know this guide may be a little weird or be to stretched out with no flavor but this is my first guide so feel free to tell me how bad it was etc but every game should be a planned out experience if you truly want to do all you can to win.I feel if people spent more times planning their lanes or playstyle league would be less toxic and people would find it harder to tilt but that's just an opinion anyways thanks for reading this guide and GL .