That is not being vain. It is bodybuilding. Posing to achieve a presentation of full definition takes practice. That means spending time in front of the mirror and checking EVERY little fiber of every muscle. That's what the judges are looking for. Any sort of sign of a muscle group that is lacking and not proportionate to the rest of the muscles will cause point deductions. Posing is one of the most important parts of the sport, and has nothing to do with ego or being "vain".