Vespers is one of eight playable map zones in The First Descendant. When you start playing, it will be locked until you meet specific requirements. Here's how to unlock Vespers in The First Descendant.
How to unlock all regions in the first descendant (Echo swamp, vespers, magna desert, white-nite)
To unlock Vespers in The First Descendant, you must complete all main story missions in Sterile Lands and defeat the Executioner in a Void Intercept battle.
This is also the standard way to unlock all upcoming zones in the game. Clear the newest zone, complete the new Void Intercept battle, unlock a new zone, rinse and repeat.
Luckily, you don’t have to keep a checklist of all of these things, because you can just follow the purple quest marker, which will already guide you through all of the zones in the game. So if you’re not sure how far you are in the story, just follow the purple quest marker.