Hello Pan
I tested vchanger some time ago, but I have not tested in production.
Always you create a volume is necessary to run in bconsole "update slots" and "label barcodes"
I understood that you have 4 hard disks ,so you need 4 magazines and 4 Devices in bacula-sd.
I've wrote a article to configure bacula with vchanger in portuguese brazilian.
I've created a script to create volumes with better labell instead of vchanger default.
You can use google translator.
I gues this can help you.