One thing to consider in all of this (and on the show as well possibly) is that I find it odd that werewolf venom works on vampires in the first place (it works on Originals too).
This may mean we are overlooking a VERY important factor concerning werewolves. Vampires were created under a certain setup, with VERY specific weaknesses...even more true for an Original.
The fact that this venom KILLS vampires and sickens Originals makes me think some odd things...
It's possible that the venom is just "magical" in nature and therefore it works on vampires because they have no resistance to magic. That is the easiest answer and more than likely the most logical for at least the TV show (I wouldn't use it for fan-fiction, because I think intricate answers are funner and are also better; logic and detail are very important in a very good book or in writing; an example of this: George R.R. Martin's book series, "A Song Of Ice And Fire", also known as "A Game Of Thrones"...and if you haven't read it you are doing yourself a disservice). Though it is possible that it is just a chemical venom, completely normal, but it just has the right ingredients to kill a vampire...which gets to my next point...
But, another option is that their venom is somehow a type of toxic/venomous chemical (I'm one of those people that actually wants to know if it TRULY is a venom or a poison; could you drink it as a vampire, for instance?) that has an effect close to extreme vervain poisoning and finally it kills the heart in the same fashion a stake would (or wood). This could mean the venom has origins related to those two sources of weaknesses that vampires have always suffered from. Perhaps their bodies take in chemicals and then reproduce the chemical setup found in vervain through a "venom organ" (a complicated explanation for what could be going on, but essentially you get my point--they've created a venom that has vampires uniquely in mind). This of course is more of a "fan-fictional" type of idea, comparative to the fire-breathing organ in the 1980's fantasy movie classic, "Dragonslayer" (you'll know what I'm saying if you've seen the show; if you haven't seen this movie, DO!)...
The next possibility is that at a certain fundamental level, werewolves AND vampires are the SAME creature...from a magical structure standpoint. At some point though someone used the moon to power the wolves, and Esther used the sun for the Original's recipe... But, other changes are a possibility too; like instead of "white-oak trees" for an Original, and werewolves got "granite rock" (as it obviously isn't silver in this series, as it doesn't hurt them...BUT, if we EVER see a werewolf get hurt by something strange--like a sandstone rock embedded in their flesh--then we'll know what an ingredient for the spell most likely was) and wolf's bane happened to grow at the base of the rock, etc...
That would explain why the venom IS a killer for vampires, because it IS essentially something akin to "toxic blood" for them. Vampires and Werewolves were created, accidentally, by a spell that was extremely similar...only the ingredients changed; making them perfect enemies. This would further explain (if you wanted to add more logic to the fan-fiction or if the TV show somehow falls into this setup--part of it may be true...) why Klaus's blood cures werewolf venom too.
Anyway, I thought I'd throw these explanations out there...I lean in favor of the last idea the most, however...
I edited this because some didn't understand exactly what I was saying or what I was getting at, I think. I think misunderstanding the line between fan-fiction, TV show ideas, details, and knowledge having nothing to do with either. I edited this post to completely clarify the ideas within (for intake or for use elsewhere by anyone that so wishes...).
If you think I'm thinking this through "too hard", I'll assume you mean that you mistook elements I meant as fan-fiction type material for ideas (and you simply don't understand where I'm coming from mentally and I needed to spell that out a tad better; don't worry it doesn't bother me, until you start to insult or try to mock me when I've done nothing to you) or thoughts that the writers of TVD personally hold or have opinions on for use in the writing of story elements. But, I have quite a bit of scientific knowledge to begin with, on top of that I'm used to reading a lot of books (many with a lot of extremely selected details, hidden away and slowly revealed for the benefit of the audience--to show the depth of thought and process used by the writer to create their works), then I also write some fan-fiction that indeed includes these type of elements...complex ones--though carefully explained by a character (not by "me" in a side-note)...