faction issue

[RedDrag0n0]

so i accidentelly joined the stormcloak rebellions unintentionally and i was wondering is it possible to join ther imperials and leave the stormcloaks ive only done the ice wraith quest so far

sidequestion: im a werewolf in skyrim but im gonna purchase dawnguard i was wondering can i still be a vampire there because before dawnguard they were plopsty​

 

[Arn Magnus]

Why did you do the quest "joining the stormcloaks" if you didn't want to join them? It really couldn't be labelled any clearer. But no I don't believe you can switch sides.

 

[The Fatalist]

SPOILER: Yes, you can. When you are given the quest to retrive the jagged crown (you'll know when it happens) you can take it back to Tulius instead of Ulfric, effecively changing your side.
But that is your last chance. Go further than that quest, and you are stuck with that faction.