when I get a gluten free meal, any packaged product comes with a list of ingredients; but that is a special meal.
in the past, when my gluten free did not make it on board, the flight attendants could not tell me what was in the other meals.
If you are very allergic or sensitive then don't trust anything that is served unless it is packaged and marked clearly as free of whatever you are sensitive to or whatever you choose to eliminate from your diet. Otherwise, just eat what is obviously something you can eat.

As somebody else said, vegan is usually vegetarian with no animal products (eggs, milk and their derivatives)