Yes I think yours is a vodafone branded/locked handset. What I meant by unbranded was that the handset wasn't supplied by a mobile provider and isn't locked to one provider. I think yours is the vodafone one with the added vodafone bloatware. Sometimes the software updates come out later for those phones because vodafone etc add their own software to it and test to make sure its working properly.
According to sammobile though the vodafone lollipop update has been available for download via Samsung Kies since 2nd April.
http://www.sammobile.com/firmwares/d.../GT-I9505/VOD/
Go to settings-My Device-About phone-Software updates and check for update.
If that doesn't work install and run samsung kies on the PC and connect your S4.
http://www.samsung.com/uk/support/usefulsoftware/KIES/
Apologies if you already know how to do this.