Most of the major airlines will provide alternate meals on meal-flights upon request, if the request is made 24 hours in advance. The low-cholesterol meal will still include some fat. Sometimes the vegetarian meal has less fat than the low-fat meal. For example, you might get chicken and margarine instead of beef and butter. Be sure to say whether you mean vegan or ovo-lacto, and be prepared to explain the difference to the travel agent.
Some caterers think that a vegetarian meal means a meat meal with the meat removed.
So be prepared for disappointments. When traveling overseas, the words carry yet a third interpretation, with vegetarian meaning vegetables, and nothing else. You may wind up with better luck asking for a fruit platter, which are usually very good on most airlines. The Kosher meals are glatt and double-sealed. Wilton Caterers is the largest supplier of these meals, although there are a number of smaller companies as well.