How can I make it easy for the waiting staff to identify the vegetarians at an Office Christmas Party?

Think its gonna have to be up to the vegetarians. Have some Christmas stickers - kinda big - at the party and when they are ready to sit down they have to grab one and stick it on the back of their chair.

They can serve the vegetarians first, bringing out the plates and approaching each table with the question..."Who is having the vegetarian entree?". After all the vegetarians are served, it'll be easy to identify everyone else by who doesn't have a plate of food.

Name the steakhouse and ask what they have on their menu for vegetarians. Then ask your boss if there would be a concern with the 4 of you ordering from this menu. I am guessing that you're only going to be given a couple of selections to opt for from. If it can be an open menu, then I suppose there will have to be whatever, just ask.

Just get the waiting staff to ask everyone their preferences I.E. Vegetarian, Vegan, Any allergies?

I think you could give them a card or something to tie on their chair, I mean most vegetarians are pretty worried about being served something scary, so I'm sure they wouldn't lose the card.

Tie a ballon to that person's chair. Then waiters would be able to tell from across the room how many of those plates he will need.

Have ready made cards in the middle of the tables, tell people if they require a veggie meal they need to place one of these cards in front of themselves. They cant forget what's already there then!

Ask the vegetarians to come naked and smear themselves in excrement. That should do the trick.