Iberia does offer a vegetarian menu. Just select the "vegetarian option" when booking at Iberia.com or through your tavel agency. Your request will be recorded and Iberia will supply the meal to you on board.

Just to skip problems, mention your request at the check-in counter. The agents normally take this into account even if the passenger doesn't say anything and command the meal to the catering subcontractor, but remind that to them is good a reminder, particularly when they are tired or they have too much work and they go that fast that they could skip something requested.

Enjoy your flight!