In /etc/fstab/ include a x-systemd.requires=unlock-bcachefs-vault.service mount option. This service does the unlocking before the mount is triggered. Example line as in my /etc/fstab:
UUID=bf512699-6643-4d96-a793-daaf3f1d34f1 /mnt/vault bcachefs nofail,x-systemd.requires=unlock-bcachefs-vault.service 0 0
The content of the unlock service: