XenForo is shaping up nicely. A warning system is an absolutely essential part of the mod workflow for the large forums.
I'd like to second the ability to create a thread on each warning in the designated forum, with warned post content quoted. Otherwise it's hard to keep the track of the moderators' actions. Also, the user can edit his warned post and the original warning intent may appear confusing.
On a related note, I'd like to discuss the warning system concept and the moderator workflow when using the warning system.
I see no merit of warning an offensive post without any incentive for action on the part of the warned member. Ideally, he should be made aware of the rule he's breaking, and given a chance to correct his misdeed in the warned post. Another reason is that the moderators do not scale - while in the smaller communities it's possible for them to manually edit out all profanities/trolling/personal attacks in the warned posts, in the large forums it's simply not possible without growing the moderator team proportionally to the growth of post traffic. It's much simpler and effective to nudge the members to do the editing themselves.
However, the existing warning systems (be it vB's or XenForo's) offer little help for that. Once the warning is issued, the member can do nothing about it, even if he edits his post. He may not even have that chance if his post is deleted in the process. If he edits the warned post, there's no way to inform the mod about the edit except for PM (and that doesn't scale). Then it's completely up to the mod to manually track down the post edit and reverse the warning if the offensive part was removed.
To force the members do the actual editing work, the warning should be removed on the warned post edit, and the members should be made aware of that feature. And to check the edits (and for the accountability), a thread should be created in the designated forum for each issued warning, with quoted post inside. The warned user is informed about the particular rule he's breaking via a templated PM and is advised to edit his post to remove the warning. Once he does that, the warning is removed, and the edit note with new post content is added to the thread that was created on warning. The moderators can now easily see if there was an edit since the thread is bumped on top, and check if the post content is ok now.
If the offensive content is still in the post even after the edit, the mod can elect to issue a "hard" warning (vs the "soft" warning before), denying the member his ability to remove the warning after the post edit.
A good further incentive for the member to edit the warned post is to hide the warned post content from public view, replacing it with a placeholder (we use "This post is temporarily hidden while <username> edits it"). While it doesn't sound too humiliating, the appearance of such placeholder is a common knowledge (at least for the forum regulars) that the associated post is warned, so it's an additional stimulus for the the post author to edit it.
We've been using the system with the tweaks described above with great effect for a few years already. I also
posted about it in the original suggestion thread.
tl;dr - please add the ability for a warning to go away on the warned post edit.