The following errors are shown when using the Altiris Log Viewer:
Failed to load and initialize product listing data
SimApplicationContext failed to initialize. Product listing failed to load
Unknown at this stage.
Symantec have fixed this issue in version 8.5.3701 of SIM.
Using REGEDIT, browse to HKEY_LOCAL_MACHINE\SOFTWARE\Altiris\AIM\Configuration\ProductListings. Below ProductListings there will be folders '0', '1', '2' and so on. Remove only the numbered folder(s) that are specific to the Veritas product listing file (select the folder and view details of what it corresponds to in the right-hand pane).
Once folder(s) has been removed, restart SIM and it should prompt you to update to the latest version (at time of writing, this is 8.5.3701). Once SIM has been updated, you should be able to select and load the VSR-MS product listing file and proceed with install/upgrade.