Ok,I became a stormcloak and we conqeured whiterun.
So now Vignar Graybeard is Jarl.
The problem is I need to trap a dragon in Dragonsreach and he's not there.
If i just don't talk to him and I just go and call the dragon it won't come down.[yes,I have the call dragon shout]
How Do I finish this quest?
also I used a cheat to make him appear but he had no dialogue about the quest...

I'm in the same part as you, so I can't help.
I'm about 110 hours in the game and just joined the Stormcloack yesterday, I immediately felt bad after going to Whiterun to destroy it!
I think I'm going empire now... seems stupid, but I'm not a betrayer.

You need to post this in the "Skyrim Cheats, Hints, and Spoilers" forum.

He wanders about Whiterun. I've seen him down in the market square a couple of times. If you activate the quest in your journal, the quest marker shows you where he's at.