If everyone became vegan we'd only see farm animals in zoos because there'd be no need for them elsewhere. We'd go to safari parks to drive through majestic herds of cattle and sheep. Doesn't give the same tingle of excitement as majestic herds of wildebeest, does it? 
If humans disappeared all together then nature would simply take over again. Our cities would crack and crumble and eventually disappear under the greenery. Within a few hundred years it would be as if we'd never existed. Then evolution would start the process all over again and one animal (probably an ape of some kind) would evolve to a higher level and the road to civilisation would start all over again.