One less animal isn't killed. They're not like "oh no guys someone went vegan, spare this cow". They'll kill the same amount, and it's just more meat for the rest of the world.Why should a police officer even bother stopping a robbery ? It's not going to stop all crime?
You avoiding meat DOES do something. One less animal is killed.
Another thing is, more and more people are going vegan, and I'm sure this collective increase has done something indeed (coupled with the overall increased awareness of individuals to become health consious in general)
Plus you're killing plants too which leads to less photosynthesis so in the end you're killing humans by reducing the quality of the air