The macro uses case statements so if any of those statements is true, the test will be true. They don't all have to be true.
If you want to check the To field also then you will need to add the three checks e.g.
Case InStr(1, .CC, "gmail com") > 0, _
InStr(1, .CC, "msn com") > 0, _
InStr(1, .CC, "outlook com") > 0, _
InStr(1, .To, "gmail com") > 0, _
InStr(1, .To, "msn com") > 0, _
InStr(1, .To, "outlook com") > 0, _
InStr(1, UCase(.Subject), "ST26") > 0
Set olFolder = Session.GetDefaultFolder(olFolderInbox).folders("Vendor")
olItem.Move olFolder
olItem.UnRead = True
If the Move doesn't work then either the Vendor folder doesn't exist or it is not where the macro expects it to be. The macro looks at
Set olFolder = Session.GetDefaultFolder(olFolderInbox).Folders("Vendor")
i.e. A direct sub folder of the default InBox. What is the path of the folder?