Are the vegetables actually acquiring moisture from the process?
Arguably the steam is simply conveying the thermal energy to cook the product (whereas oil affects texture, taste, nutritionals), so it'd be analogous to e.g. declaring photons as an ingredient in a product you heated in a microwave, which isn't something I've ever seen before 
Always worth checking what the big retailers are doing with similar products, as you can generally rely on them having had a big team of regulatory people go over things. FWIW I can't readily see any cooked veg examples where the water is declared as an ingredient.
Maybe potato/veg wasn't the best example (although I'm fairly certain that they would take up some of the water)
A clearer example would be rice or Cous Cous (both of which we use) - these very clearly take up water, so, the water should be declared something like this?
"fish(60%), cooked basmati rice (30%){basmati rice, water, salt}, soy sauce......................etc