I would try and keep it as simple as possible.
Only thing I would consider splitting off in your setup is /opt/zimbra/backups if you are are using the network edition. They have the potential to get very large and having them on separate storage with a separate quota/partition size would be very sensible.
To get storage into your OpenVZ container, look up how to bind mounts:
Bind mounts - OpenVZ Wiki
So you mount /mnt/slowstorage in the host, then re-bind mount it into the container.
But this is only an issue with the network edition and if you have not enough space for the data and all the backups.