my ILs just "didn't get it" and in some ways still "don't get it" in regards to vegetarianism.
my MIL would try, honestly she would, but it would always fall short (because she doesn't understand the 'theory' of vegetarian cooking) and then she'd feel bad (and i would be hungry). my husband and I do bring a lot of our own food, because we also eat organic, etc, and they don't--but this also upsets them.
i've found that the best that i can do is to do things 'pot luck' style. my MIL is in charge of the main course and dessert, and i often bring a salad. Sometimes i also bring soup and/or a side--though often my MIL will make a completely vegetarian side dish (usually starch).
my MIL got really "into" this style of doing things and started asking my SIL to also bring things when she came over. she typically brings some kind of dessert or the other side dish (usually starch).
So, our meals are typically like this:
Me: i bring salad and/or soup
MIL: meat, side or dessert
SIL: side or dessert.
this way, i typically 'get' to have salad and soup (if i bring it), the side, and sometimes the dessert.
i tend to avoid margerine of any sort--and my MIL and SIL bake with it. So, baked desserts are out. They also buy commercial frozen yogurt and ice cream, which typically have high fructose corn syrup, also something that i avoid. So typically, i forgo dessert (saying that i'm full).
I bring enough, of course, for everyone. This has worked out really well for us. maybe it will work for you!