I think sometimes the problem is people request the 'wrong' type of vegetarian meal. Whilst there are several different variants of vegie meals sometimes people request a VGML (vegan) instead of a VLML (lacto-ovo) so they're expecting to receive a meals without meal but instead receive one without any animal products at all.
I heard of a case where a pax was requesting a vegetarian meal and when asked by the airline if she wanted a vegan or lacto vegetarian meal she said "what's that"? Wouldn't a genuine vegetarian know that?
Vegetarian Meal (VGML) – VEGAN
Also known as Vegan, this meal is totally free of any animal products or by-products such as eggs or dairy products. It contains one or more of these ingredients: all types of vegetables and fresh fruit. It does NOT contain any type of meat, fish, or animal products or by-products.
Vegetarian Jain Meal (VJML) – VEGAN
This meal is for members of the Jain community who are pure vegetarians. It is prepared with a selection of Indian condiments. It contains one or more of these ingredients: fresh fruit and stem vegetables that grow above the ground. It does NOT contain: animal products and by-products, and any root vegetables such as onions, mushrooms, ginger, garlic, potatoes, carrots, beets, radishes, etc.
Vegetarian Oriental Meal (VOML) – VEGAN
This is a Vegetarian Meal (VGML) that is also prepared Chinese or Oriental-style.
Vegetarian Lacto-Ovo Meal (VLML)
This is a Vegetarian Meal that may also contain eggs and dairy products. It contains one or more of these ingredients: vegetables, fresh fruit, eggs, dairy products, and pulses. It does NOT contain any type of fish or meat.
Asian Vegetarian Meal (AVML)
This meal is available for vegetarian passengers. It is usually aromatic and spicy, and incorporates flavours from the Indian sub-continent. It contains one or more of these ingredients: all types of vegetables, fresh fruit and milk products. It does NOT contain any type of meat, fish or eggs.
Raw Vegetable Meal (RVML) – VEGAN
This meal consists exclusively of raw vegetables and salads.
Fruit Platter (FPML) – VEGAN
This meal may be ordered for dietary reasons. It may also be ordered by members of certain communities who eat only fruit while fasting. It contains one or more of these ingredients: seasonal fresh fruit. It does NOT contain canned fruit.
Airline Meal Codes for Vegetarians & Vegans « Vegan Nomad Guide