When weighing questions such as these (and they can apply to other dietary restrictions besides vegetarianism), we like to remind ourselves that our goal – as host or guest – is to do our best to make the other person feel comfortable. It might not be possible to eliminate all awkwardness, but genuine thoughtfulness and respect go a long way.
In an ideal situation, the host and guest are already well acquainted and any discussion of vegetarianism is unnecessary. The host, whose duty it is to ensure that guests are happy, plans for a meatless option (either a flexible main dish or a substantial side) and the guest remembers to thank the host. But what happens when the parties are less acquainted or – uh-oh – the host forgets?
A lifelong vegetarian, I personally prefer to keep things low-key. If the gathering is large and/or buffet-style, I stay silent and simply make do with what's available. This occasionally means going hungry, but I'd rather not make a fuss and then raid the refrigerator when I get home.
However, if the dinner party is a smaller affair where avoiding food would be obvious and possibly hurtful to the host, I take a cue from the Emily Post Institute, which advises that a vegetarian respond to an invitation with something like, "'Thanks so much for the invitation. I should let you know that I’m a vegetarian. I’d love to bring a quiche if that’s okay with you." This way, the host is both informed and relieved of the pressure to cook an alternate dish. The only thing I might do differently is ask the host whether there is a particular dish I could bring that complements the rest of the menu. Alternatively, I might emphasize that I am perfectly happy eating side dishes, but that I wanted to mention my vegetarianism ahead of time to prevent any awkwardness on the night of the party. Most of all, I try to be considerate and never demanding. (Also, I would never bring a side dish unannounced.)
If the vegetarian issue has not been discussed until the night of the party, or even if it has but there isn't sufficient food to eat, the vegetarian should make an effort to tell the host how delicious and adequate the other offerings are. For me, focusing on the host's good intentions and the pleasure of good company prevents me from ever feeling disappointed or resentful. I also think it is important not to even talk about vegetarianism at the table unless the conversation is initiated by the host. Regardless of whether one's vegetarianism is for religious, health, or ethical reasons, bringing attention to it could make the host feel bad.
How about you? How do you handle such dinner party situations as a guest or host?