Special meals, which includes vegetarian, need to be ordered in advance. Airlines don't cater flights with vegetarian or other special meals "just in case" a passenger might request it. If you couldn't find where to order the meal on the website a simple phone call would have done the trick.
About your other "complaint" Air France and Delta are partners in the SkyTeam alliance. It's very possible that the Delta flight you were on had a codeshare flight number for Air France. Airlines and other travel websites are required to show the operating carrier when listing flights. Had you bothered to read the information fully, you would have seen something like this:
Air France flight #xxxx (operated by Delta Airlines)
It would also have been listed on the itinerary confirmation you received but again you probably didn't bother reading that either.
Remember commercial aircraft are not restaurants. Airlines provide food and beverages as a service to their passengers but their primary responsibility is to transport you to your destination safely. Even when pre-ordered, mistakes can be made and a special meal might not be loaded on your flight. If you have special dietary requirements I would suggest in the future bringing along some food that you can consume should the food offered by the airline not meet your needs.
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