vfat does not have permisions and ownership! You can't set any permanent info on the partition itself, only via mount options.
You can impose ownership with the uid/gid options and permissions with the umask option.
You fstab should have a line like this:
/dev/hdb1 /mnt/vfat_usb vfat umask=077,uid=1000 0 0
In this case all files on the disk will be -rw------ for the guy with uid 1000. Hope this helps.