1 - open the vcx in the class browser (the class browser presents classes in a library in alphabetical order)
2 - open each class (double click) and save it (Ctrl+W) starting with the class that you want to be the last one, going in reverse order, to the class you want at the top of the toolbar.
The order won't stay, every time you modify a class in a library, it will go at the bottom of the .vcx file and... at the top of the toolbar.