Are you sure you need to request a vegetarian meal? I stopped requesting vegetarian meals, in part because of the reason you mention, but also because I figured out I could get better meals that are still vegetarian - at least on United - without ordering them. It's possible this could work for you on the airlines you fly also.

The typical vegetarian fare on United (or any airlines I've flown for that matter) in coach and business is crummy. What's worse, they tend to think vegetarian means vegan. While everyone else on the plane gets chocolate cake or some other luscious dessert, I get the horrid Zen 'N' Now cookie. This cookie lacks eggs, sugar, butter, oil and taste. It's basically what I'd expect a mouth full of sawdust to taste like.

Anyway, if I get the regular meal, coach always has a pasta without meat and a green salad and business generally has a green salad and an appetizer that's vegetarian. But the best part is I get a real dessert ... and I know there's no special meal request keeping me from an upgrade.

This has been working for me and it could be something you'd want to consider.