I've just read your post again. You will be able to mount as a user if the user has r/w access to /dev/vnd* nodes.
From mount(8) --highlight mine:
Code:
...Only the superuser may
mount file systems unless kern.usermount is nonzero (see sysctl(8)), the
special device is readable and writeable by the user attempting the
mount, and the mount point node is owned by the user attempting the
mount.