I'm currently in Creative (started out in Survival), and I just flew in a single direction for the past 5 sessions (while I'm having coffee in the morning, etc). I do this sometimes when I want to gather resources from village forges, crops, desert pyramids, etc. While flying over and ocean, I came up to a woodland biome (dark wood trees and with a swamp close by), and BAM there was this huge mansion just... sitting there.
At first I thought I found a game glitch, a hidden easter egg... something! Then I googled it and found out what it was. My question, however, is this: I want to basically move into this thing, but when I go back to Survival (peaceful mode), am I going to have to constantly clear out vindicators and evokers or once they're gone, they're gone?
They only spawn when the structure is first created, ie when you first got close enough.
Although it's quite dark in there, with lots of space, so quite a lot of other mobs spawn too; you'll have to light it up a lot. But it's not so bad; perfectly do-able. Those new mobs are quite hard, but not that difficult - work through the rooms one at a time, clearing it and lighting up. Snipe from a distance with bow+arrows.
You'll maybe find some hidden chests and treasures too :-)