I have worked in service many years, including for a swish caterer who regularly did events for 800 - 1200 persons. So I feel your pain, I do agree people should let the company know in advance. Also I agree that companies can lose those communications, delivered in the weeks leading up to the event (there are always late comers!).
All of that said, it is the caterers job to, y’know ‘cater’ to the clientelle. That means knowing that out of 800 persons, a certain percentage will want veggie, vegan, halal, kosher, food allergies etc, without any warning.
We never did an event that we did not have a couple of trays of beautiful vegetarian/vegan food in the oven ready to plate if needed, (and it always was).
It is the servers job to ask the person requesting a special meal, “Did you order a special meal?”, if the answer is ‘no’ then it’s the servers job to convey that they will try their very best to accommodate the request, but those who special ordered will be served first. Gently, politely and kindly - no attitude. The server should also ask that person if (veggie prepped meal description here) is acceptable?
The chefs I worked with never had a problem, as this had been anticipated. It was a simple thing to return to the kitchen and ask for a veggie meal to be plated hot from the oven. Does it slow down the service to the veggie eater? A little, but they are understanding if they then get an edible hot meal.
Were there times we were caught without enough veggie meals? Very rarely but yes, but when that happened the people left out were not those who had pre ordered, and apologies were made while trying to accommodate, best we could, with whatever ingredients we had with us. Our customers were unfailingly appreciative of our efforts.
As for tight food costs I just have to say it’s the nature of catering that you have to be prepared. Think of a buffet, you can’t know exactly how much food each person will take, yet you have to have enough for everyone. It’s all about percentages and experience, if they can work it out for a buffet they can work it out for veggie meals. The truth is, especially in convention centers/cafeterias/hospitals etc, they just don’t want to be bothered. You can see why it’s so much easier if everyone just has the same meal and they count on that when they price the event for you.
Catering is about giving the customers what they need and trying to anticipate their needs. If this situation plays out again and again, as you’ve described someone is not doing their job.
In a perfect world persons with special food needs would let the caterers know well in advance of the events. It’s not a perfect world and people are people, to expect otherwise is to ignore the obvious.
Mind you this was not a convention center it was a top end catering company, there is less room for disappointment when people are paying top dollar to be sure. People do need to keep their expectations in line with the venue.
And I just have to say, the day I went to a table and asked them to raise their hands to indicate what they ordered would have been my last day working for the caterer. 