Isram gave me this quest to take out a vampire disguised as an advisor in a jarl's court. I talked to the Jarl and he demands evidence that this man is a vampire before I can take action( i guess the glowing eyes aren't enough!). The only evidence i could find is a letter on the vamp himself. The problem is, I'm a terrible pickpocket, and have a real low chance of taking the letter without getting caught. Anyone find another solution to this?

You get a 25% bonus to picketpocket if you are hidden.So sneaking will help a bit. Or if you have the alchemy perk to poison people, you can give them a paralysis potion. When they drink it they will fall to the ground. You can't interact with them but as soon as they start getting up, loot them and the game will treat it as looting a dead body and you will have 100% success rate even though the pickpocket % is still low. You have to be quick though as they are getting up or it may not work so save before hand.

Kill him and trololo back to Fort Dawnguard with the town guard on your heels.

Yeah, I wasn't crazy about having to pickpocked the bugger to get the evidence. I saved beforehand, got a 55% success rate and took it without any issues. A bit of a weak quest in that this all happens in the jarls longhouse. It would have been nice to tail/follow the dude around, etc, to gain the evidence.

fortify pickpocket potions?

If you kill him in a secluded area you won't get the bounty and you'll get the letter.

Enchanting items and potions can raise your pickpocketing.