I'm a Vegetarian (not Vegan) and usually order the Hindu meal on domestic. I think Indian Vegetarian is "Asian Vegetarian" on LH. I'm not sure UA has a specific Indian Vegetarian option (although the Hindu meal has always been equivalent).
My UA Hindu meal has always been vegetarian and come to think of it Vegan for the most part. I am fairly pleased with what I get on domestic and to/from LHR. Also it's WAY better than the miserable options I had on American the one RT I did transcon with them.
It's usually not bland (except if it's a rice pilaf, and that's not too bad), and they also give you dessert (Indian cake called Halwa). It is usually quite difficult to pull out of the plastic wrapper it comes in, but after that, it's pretty good!
On the slightly shorter flights (SFO-ORD or shorter), you usually end up with some "just fruit" stuff and perhaps eating veggie parts of the std meal may work out (I still stick to the Hindu meal).
Hope this helps.