Note:
First, go to the source site > WPvivid Backup Pro > Export Site > Export to Localhost(Web Server).
Choose the option "For the purpose of moving a subsite to a single install".
Enter a subsite title, or click Search directly to show all subsites, then select a subsite to migrate.
Go to the target site, go to WPvivid Backup Pro > Import Site > Import from Localhost(Web Server). Click Select Files to select the exported zip(s) or just drag and drop the zip(s) to the field.
Note: Please make sure to upload all the parts(zips)(if any) of the backup at a time.
Once done, click Upload button.