So, I've found a solution (debugger thank you).
I have seen that the procedure LoadResourceImagesFromFile in cCJCommandBarSystem.pkg sets the directory for the styles.
So, I have copied this procedure in my own subclass and changed in in the way, that the styles are searched in the program-path for users, who haven't installed the client and the ResourceImagePath for the programmers, who have installed VDF.
The source-code for this procedure now looks like this:

    Function LoadResourceImagesFromFile String sFile String sIni Returns Boolean
        Handle hoGlobal hoRes hoWorkspace
        Variant vGlobal vRes
        Boolean bOk bFound
        String sDir sProgramPath

         Get phoWorkspace of oApplication to hoWorkspace
        Get psProgramPath of hoWorkspace to sProgramPath
        Move (sProgramPath + "\Styles") to sProgramPath
        File_Exist sProgramPath bFound

        Get Create (RefClass(cCJCommandBarsGlobalSettings)) to hoGlobal
        Send CreateComObject of hoGlobal
        If (not(IsFileNameQualified(sFile))) Begin
             If (bFound = True) Begin
                Move sProgramPath to sDir 
            If (bFound = False) Begin
                Get ResourceImagesPath to sDir
            Move (sDir + "\"+ sFile) to sFile
        Get ComResourceImages of hoGlobal to vRes
        Get Create (RefClass(cCJResourceImages)) to hoRes
        Set pvComObject of hoRes to vRes
        Get ComLoadFromFile of hoRes sFile sIni to bOk
        Send Destroy of hoRes
        Send Destroy of hoGlobal
        Function_Return bOk
This works fine.