It should be in the backup of the domain that matches that ID. Where exactly, I’m not sure off-hand. There are several layers of tarball, though, so you can’t just gunzip/untar the top level of the backup and expect to find individual files.
Also note that there have been reports of some Windows tools for working with gzipped tarballs failing to do the right thing with Virtualmin backups…being unable to extract everything. I think Jamie made changes to try to work around those buggy implementations, but I’m not confident that’s completely resolved. Best to use the GNU implementation of tar and gunzip. And, if you’re not sure what tools to use for a given archive, file can tell you what’s format it is.