This page of our guide to Psychonauts 2 lists the Memory Vaults available in Lucrecia's Lament.
Two Memory Vaults can be found in this level.
The Deluge of Grulovia
Location: Flea Circus.
It's very simple, head to the right corner of the room, just opposite of the carousel. Hit the Memory Vault a few times to add it to your inventory.
Nona's Call for Help
Location: Nona's Quilts P1.
You will find the Memory Vault in a small forest, right next to the exit from this part of the area.