OK, new VirtualMin Pro user here, just set up on a new box and about to start migrations. ServePath support writes, in response to my request to back up the domains one by one at a time starting with EastKauaiWater.org:

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The backup of the domain eastkauaiwater.org has been placed in the new server 64.151.103.88 within the folder "/home/pleskbackups/". You will find two backup files

backup-eastkauaiwater
eastkauaiwater-backup-agent

The first file "backup-eastkauaiwater" is the backup of eastkauaiwater.org taken using the default plesk backup utility. The second one "eastkauaiwater-backup-agent" is the backup of eastkauaiwater.org taken using the backup utility agent provided by plesk for plesk 8.0
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I've ask them the question: what's the dif between the "default" and the "agent" versions.. but I thought I might post this here to...

Any ideas on which file to pull inside the VirtualMin Migration tab?

I have no idea what the difference is. When I try to parse the sentence above I come up with "This backup was made with Plesk, and that backup was made with Plesk." Obviously, they intended something different, but I don't quite grok what the difference is.

I would hope we can handle whatever kind of Plesk backup file you can throw at us, but we only know what we've seen...we have to reverse engineer the format, as it is not documented anywhere (which is understandable, I guess...ours isn't documented anywhere either, but ours is a tarball with a few special files included that are pretty intuitively named, so I think just looking at our backups will make sense...Plesk uses a weird MIME encoded wrapper in addition to regular archival tools).

Anyway, try one and see what happens. It probably won't cause your server to explode, even if it fails to work.