Actually, my friends and I have been talking about exactly the same thing. In the 80s we were being told in school that Europe might have to adopt a 'one child per family' policy like China before long. Instead, the governmnet encourages people to have many children because they want a high population to support the glut of people who are now retiring
Then we have a viscious circle where we need more people working to support the health and education systems, and all the stay-at-home-Mums claiming welfare
Going vegan will help climate change, but the figures will depend on just how fast the population grows and how much energy they consume. For example, building affordable family housing in cities instead of in suburbs and villages will slash carbon emissions from cars. Initiatives to give all children computers, for example, will increase carbon emissions. The issues are very complex.
I would certainly like to see families be encouraged towards responsible family planning, and a little more support and respect for people who remain childless. Even if everybody becomes vegan, population will still eventually become an issue again.