As others have said, the VGML, AVML, VLML, etc are completely hit-or-miss. Worse, you do stand a very real chance of the meal not being catered on your flight (I had that issue several times last year. The FAs felt bad and tried hard to make up for it. Kudos to them!)
My pro tip: Board early and, as you're boarding the aircraft, right in the doorway, as you greet the FA, casually ask if the FA is the lead. They'll usually point you to the lead FA. introduce yourself and politely explain that you're a vegetarian and will be sitting in seat 6W (or whatever it is) and could they hold a meal for you. They'll definitely be accommodating and make sure that one meal is set aside for you.
Note: This assumes that you are an "easy" vegetarian and can eat one of the meals on the aircraft, as long as it doesn't have the meat on the entree. (For example, on things like the "Salmon Salad", I have no idea whether the Salmon is actually on the salad when it's loaded onboard the aircraft and they just remove the Salmon from the salad before presenting it to me, nor do I ask). I'm also ok with dairy products and egg-based products. If you're stricter, you may prefer to order a meal and, if it gets messed up, just eat around whatever you can off the tray (there's usually SOMETHING on there that's vegan-friendly, although not much), and they'll usually grab you the vegan-friendly snack box (the one with humus) from in back.