To the OP, Charley:
To me, a longstanding vegetarian, but with plenty of experience in a non-conducive environment: do the best you can. In this particular circumstance, I’d say the thing to focus on is the fact that your in-laws honored you enough to make the effort to make you feel welcome. By chance, it got switched, and perhaps made you think of your commitment to vegetarianism in a different light. By thinking that you have “gone beyond” your self-set parameters, by no fault of your own, you begin to see how it’s not just an issue of self-control, but of living in the world we all gotta be in.
Was it the transgression of your parameters that made you feel ill, or did you really feel sick to yer guts? A couple of times a year, I eat meat; always on the occasion of being served food and not wanting to make a big deal of it. And I usually get in a constipated state, if not outright sick. I don’t agonize over it, though. In this country, we’re very lucky to be able to make the choice. We have a plentiful food supply, and can quite easily choose what we want to eat to survive. That’s a very graceful situation.
That said, and having worked in restaurants: nothing’s perfect. If you’re a vegetarian in the USA, and you eat out, you are going to have to be somewhat flexible. That doesn’t mean capitulating to whatever’s available: you should always make restaurants aware that they should offer vegetarian fare. But when the kitchen is in full swing, they’re not gonna have time to cater to you, except in enlightened circumstances. The way I deal with it is to be kind ( kinda the underlying principle of the whole shebang), make people aware of the terms of vegetarianism, and then, if it still don’t fly, be very grateful that you get to eat at all. Plenty of folks on his earth don’t have the grace to get their belly full.
So, Charley, don’t worry too much. But, thank your in-laws for their effort, tell them how wonderful the meal was, and when, at their table, the right plate gets passed to you, you’ll amaze at it all the more.