When I first installed VUM 4.0 I asked VMware support much the same question and got this as part of the answer:
Unfortunately, there is no way to filter downloading patches by version. All ESX (Embedded, 3.0.3, and 3.5.0) patches are downloaded by UMDS. After testing I have found that you can go into the directory of the downloaded patches and remove all the 3.0.3 and Embedded packages PRIOR to doing the export and then perform the import only 3.5.0 patches are imported. The <EnableDownloadAllUpdate> tag in the vci-integrity.xml is only on the VUM server but my testing showed this tag does not affect the import process. The only thing that changes what imports is what is was exported. So on subsequent releases only new patches are being exported that is all that is imported.
I was hoping this would be an improvement with VUM 5, which I'm just getting around to installing today.
So before every export, I go into the downloaded patches folder and remove everything I don't want to export. Crummy way to "filter", but it works with VUM 4.
UMDS 5.5 allows filtering what "supported platforms" will be downloaded (hurray!) but I don't see any options for filtering what is exported from the downloads.
If you only want 5.1 patches, you have to tell UMDS to only download for "embeddedEsx-5.1.0":
vmware-umds.exe -S --disable-host
vmware-umds.exe -S -e embeddedEsx-5.1.0