Vivera retainers: do you have to have fixed wires as I just want to have Vivera retainers on their own?

Having both gives you flexibility that you don't need to wear your Viveras every single night. But if you're a disciplined person and are certain you'll wear your retainers every night of the week, that's an option too. 
Certain types of tooth movements are more prone to relapse - talk to your dentist/ orthodontist and they will guide you.
Best wishes
Zak

I’d recommend to have the metal wire removed if you know you’ll be compliant that you’ll wear the Vivera. Easier to clean and floss your teeth with the metal wires off