Well, first off, The Chinese Restaurant was before The Stranded (the episode in which she claims to be vegetarian). So one could argue that she simply changed her lifestyle sometime after eating the Skyburgers.
However, the character of Elaine doesn't even understand the term "vegetarianism," anyway.
In The Stranded, Elaine is asked if she's a vegetarian, and she replies "yeah, I eat fish occasionally." But if one eats fish, one is not a vegetarian. People who eat seafood can call themselves one, and often do, and will argue up and down about it, but the fact is, they are not vegetarians, by definition.
That's like a recovering alcoholic saying "I have ten years of sobriety, but I get drunk occasionally."