I'm making some Korean chicken and am out of rice wine vinegar. Would a diluted lemon juice work
The recipe only calls for a tablespoon of vinegar so it doesn't form a large part of the dish.
You are going to get similar results but not the same flavor. I don't think you need to dilute the lemon juice although I would research the pH level of both, because this is the reason the recipe probably calls for the vinegar.
Lemon is a bad substitute for any kind of vinegar because the acid component of those two ingredients is chemically different. In vinegar this acetic acid, in lemon juice this is citric acid. The taste of both acids is markedly different.
When you only need a bit of rice vinegar, you can dilute normal alcohol vinegar 50/50 with water to get a similar concentration of acetic acid. You'll miss some of the flavor of the rice, but unlike lemon, alcohol vinegar and rice vinegar have acetic acid as the acid component.
Edit: Rice vinegar tends to be 4% acetic acid while normal alcohol vinegar is about 8%. That's why you need to dilute it.