The Vanish mechanic simply is no replacement for the Madam Mim Bounce package that makes up the heart of every Amethyst deck out there
Something like: here's the list of cards I possess
If you ever find yourself at a loss against huge creature or token army decks, this land is an immediate response that has your opponents making difficult decisions each turn
Nascent Metamorph becomes a copy of that creature until end of turn
However, assuming that you're building a fairly typical deck, there is a really simple way of figuring out about how many lands you should put in your deck
Water-Type Pokmon This is a balanced type, super effective against three (Fire, Ground, and Rock) while taking super effective damage only against two (Electric and Grass), on top of having four resist types