I completely agree with you that it would in fact be rather rude to change existing file-type assignments and understand your disappointment. However, the OnScript Installation is not designed to do such a thing and we are surprised to hear that it would do that.
What the OnScript Installer attempts to do is add file-type assignments for the documents it recognizes. After the installation you should be able to see the following two additions in the shortcut menu of the supported file types:
Edit with OnScript
Open with OScript
Where "Edit with OnScript" refers to loading the script file into the OnScript Editor for editing and "Open with OScript" runs the script using the native OnScript Host.
The installation does not attempt to modify or remove any other actions. In particular it should not modify the default Open action. (We recently updated the installation to not modify the Edit action because it in fact did that for some file types.) When you remove OnScript, these additional assignments should disappear.
Regarding the list of file-types we will prepare a document or perhaps a script that reports the settings. Basically they are: .VBS, .JS., WSF, WSC, .BAT, .CMD, .HTA, .TXT and .INI.
Now to the situation on your system: could you perhaps export the registry setting for the VBScript file type and post them? It would be interesting to have this for analysis purposes.
You can find the setting here: HKEY_CLASSES_ROOT\VBSFile (run regedit.exe and export that branch to a file).
XLnow - the makers of OnScript