If you install the proftpd-basic update on OMV 3.x, this will uninstall the openmediavault package (and all plugins). I haven't into it yet but I'm guessing proftpd-mod-vroot has to be recompiled for this version.
See this post for how to update to OMV 4.x - omv-release-upgrade failed (apt-get update failed, too)