While Veigar could be seen as an unusual pick for support, when you consider his moveset and his ability to scale very quickly (given that you know how to exploit that about him), he can easily become a top tier aggro-type crowd control support in any game that needs a bit of additional carrying. That being said, in order to avoid being perceived by your teammates as a troll, you'll have to do everything in your power to show how much explosive power this little fella packs and, above all, how you can help carry your team to victory. Here's how you can do that (it's not that hard I promise).

First, poke a lot with your Q. You should really focus on that since you can't stack your passive with minions as the ADC needs them. Don't be a bad support and only SECURE minions with Q or W, when you're sure your teammate won't get them or when they're away. This is especially good with ADCs who can roam, as you can solo farm easily and stack in the meantime.

Second, use your W when the ennemy is stuck in minion waves or in your cage, or when an ally stuns them. I would avoid using W too often, even in the midgame, because it costs a lot of mana for very little return (you won't just bink a shot on the enemy with luck alone above bronze, they'll have more than enough time to dodge).

Third, use your E a lot. This is why Veigar is an excellent support and why you can scale up as much as if you were midlane. In the beginning, only use your E as a form of defense or if you see a very good offense opportunity, because it costs a *lot* of mana. This is why you gotta rush a mana item after your support item. Then, when you have enough mana to sustain, spam E.

The obvious combo is the same as usual: when your E hits, W > Q > autoattack, and you can even ignite before W if you're in range (and it's up). If that's not enough to kill, immediately walk away (as you lack movement overall).

I'll let you decide whether you "secure" kills with your ult or if you pop it at 75% HP to help your ADC get the kill... ;)

That's it. You know the art of playing Veigar as a REAL support. Now go and make them hate you... NOT YOUR TEAMMATES! The ennemies who'll have to face the almighty wizard gnome >:)