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you will find the base images available and eligible for your system and contract in the IBM ESS portal, like VIOS 3.1.0, 3.1.1, 3.1.3 and 3.1.4 etc.
All other levels you can download from IBM Fix Central as an update package to one of these base levels, and can bundle them into an ISO instead of a bunch of separate bff packages as one of the download options.
This might not include any version known but you will find the latest fixpack version for any of the minor versions.
VIOS FP 18.104.22.168 would be available here.