Assuming you are asking for Economy Cabin. The cabin crew normally won’t let you have a look at then dish and let you decide if you take it or not. For special dishes, they probably won’t know what is inside since they load just the exact amount of special dishes which are requested by the passangers. It is very likely they will have a pasta option which is always vegetarian and you could ask for seconds (simce this option does notmally don’t run out on a good day) or for exchange of your meal. I personally don’t have experience with vegan and asian vegetatian dishes. If you are in Business Cabin, you can be out of luck for standard vegetarian options as sometimes all standard options in the Business Class menu are meat/fish options - for example for pasta option (it is not always offered) they can serve manti (turkish tortellini-like pasta filled with meat)