Vendor and model name only appear if Plasma NM has been compiled with ModemManager support and if ModemManager has already enabled the device. If neither of those conditions has been satisfied then "Installed GSM device" it will appear instead. By default ModemManager allows only root to enable devices, so I cannot enable them in the wizard. Solid also does not return my modem's vendor and model names. Maybe Network Manager can help me here.
I had to recreate the code from scratch because last month I deleted all what I had done by accident :-/
I still need to fix some bugs in the wizard and make the code cleaner. I also have thought in prettifying the wizard but QWizard does not have many options to do that. For instance, I would like to use images instead of the < and > characters, but I as far as I know there is no way to do that. nm-applet's wizard also uses a blue background in the page's title ("Choose your Provider" in this screenshot) and a frame around the widgets, it looks good in my opinion. So far I have not found how to do that neither.
I am using QDomDocument to parse /usr/share/mobile-broadband-provider-info/serviceproviders.xml, it really simplifies the code. Talking about code, nm-applet's wizard is a difficult to read code, lots of macros and options to functions, several things done "by hand", which in Qt is a simple method call.
The next step is salving the options, but I will reorganize the code first, maybe next weekend. I still need to add the code to get data for CDMA connections and some more code to hide/show the widgets that should not appear in such case (CDMA connections does not use APN for instance). There are a lot of small things to do yet, so "keep walking" :-)