You're one of those subtle vegetarians — nobody knows until it's too late. It's cool, nobody's under any pressure to disclose their dietary restrictions until the platter of cured meats or giant turkey is upon them and a decision must be made: what's the polite way to turn down meat at a dinner party?
Don't worry about manners on this one: be more assertive. That's right, if you're not willing to call just a little bit of attention to yourself, you have no business trying to be polite at a dinner party. If the host isn't aware of your vegetarianism, the embarrassment he or she will experience upon realizing there's not enough for you to eat (possibly nothing!) makes you the inconsiderate one. People throw dinners to make their friends happy, not create Awkward Situation #452. Respect that kindness and voice ahead of time that you don't eat meat, so it doesn't become a topic of dinner conversation that segues into politics, religion and your buddy's vegetarian girlfriend who nobody likes.
Also, unless you're a true vegetarian stickler — and if you were, you wouldn't be in this place to begin with, as you would have told the host in advance — consider making small compromises like eating something cooked with chicken stock. The gesture will be more meaningful than the temporary discomfort in consuming something that's about 1% meat and 99% vegetables and salt.
Bonus points: even if you weren't asked to bring anything (except wine, obviously), a vegetarian side that can double as a main course for you solves the problem before it even starts AND you get to steal a little of the cooking spotlight from the host you didn't embarrass.
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