Restoring a version 7.x backup in version 8 will result in the lose of Webmail functionality, and typically that's the only problem, but other unknown problems could occur. It's recommended that you restore only the same version backup with the installed version of VMS.
To restore Webmail follow these steps,
1. Stop all VMS services
2. Delete the the webserver.dat (v8.3) or webserver.cfg (v8.0) file in the VMS \config\ folder
3. Delete the \PHP\ folder
4. Copy all files in the \webmail\config\ folder to a safe place
5. Delete the \webmail\ folder
6. Copy all files in the \HTML\ folder to a safe place, if you customize the HTML content
7. Delete the \HTML\ folder
8. Reinstall VMS 8.3 or 8.0 depending on your current version.
After the install you should be able to access Webmail.
If you had a Global Address book in Webmail restore the address.dat file from the \webmail\config\ folder.