Originally Posted by Blackhood
Cocktail Attire is a tuxedo.
That's not the commonly accepted meaning of the term, at least in the United States, circa 2013. (Since Chicago was specified, going by American usage is appropriate. But perhaps the meaning is different in the UK, where you appear to be.) "Black Tie" would be the dress code indicating tuxedo.
Here, cocktail attire means men should wear suits, and women should wear cocktail dresses, with minor variations on these norms generally acceptable.
Hetzen, I'd suggest you wear the navy or charcoal suit, with the blazer being very much the third place choice (and "whatever pants" should not mean denim).
If you want to wear a v-neck sweater under your jacket, okay. A v-neck sweater vest might leave you more comfortable, as a full sweater, underneath a jacket, worn indoors, might result in you feeling a little warm, but that's for you to decide. Personally, I'd much prefer a "vest" vest, to a sweater or sweater vest, but whatever.
I will not address the choice of tie/pocket square/shirt, as there are just too many acceptable possibilities. Once you decide on your suit, shirt, tie, etc., feel free to post a picture of you wearing them, and ask for comment.
Obviously, make sure your shoes are appropriate and shined.
And for dog's sake, try not to over-think the whole thing. It's a Christmas party. It's not a command appearance before our rather touchy Alien Overlords, aboard their Imperial Space Cruiser. Show up dressed in basically appropriate fashion, enjoy yourself socializing with your friends, acquaintances, and colleagues. Don't get drunk and don't say or do anything which could come back to haunt you once the party is but a memory. It truly needn't be any more complicated than that.