Is it vanity to want a park in your city or a river to be clean of pollutants?
We are scared of projects like this because the scale betrays our inability to do them or perhaps fully anticipate the consequences, which is good enough reason for caution.
But vanity? A garden is never reducible to vanity, it is the cultivation of the earth and the prosperity of living things, regardless of how vainglorious the gardener may be.