Landscape
The topography of the planet would be very different. Without predators, the Earth would simply be deserted.
Diversity
The diversity of species would be incredibly scarce due to the uniformity of diet.
Mammals
It is unlikely that they would have appeared in principle, since casein (a component of milk) is an animal protein, clearly not pro vegan.
Numbers
The number of animals would have been influenced solely by climate. Rapid reproduction in a crop year would provoke the rapid destruction of the flora, as only they are capable of converting inorganic matter into organic matter.
The mass of plants required to feed the increased population of those who feed on them must increase rapidly. But this is impossible, hence the fauna would starve to death en masse.
Slowness
Evolution would be very slow. And organisms would also tend to simplify their own structure.
