Hidden list, so the row is gone. Blur, so a visible row has no last line. Quiet hours, so the phone does not light up with the message at night.
Use the decoy PIN when someone may watch you unlock the hidden list. Practice it once so you do not type the real PIN out of habit.
Agree the boundary
A discreet chat works better when both people know which photos are view-once and which ones are allowed into the vault. Guessing leads to a picture left in the thread.
Nothing here is a promise against a person who knows the PIN or who takes a photo of the screen.