Hello guys,
You can make Enpass extension work with Vivaldi browser by turning off the Code signature verification in main Enpass App as explained here in user manual. Before that, just have a look at the whole scenario and associated risks (if any, in your case) in turning off the Browser verification.
Currently Enpass supports extensions for widely used browsers; Chrome, Firefox, Safari and Opera browser. However the Enpass extension can be installed in many other Chromium based Web browsers available in market, provided they support installation of Chrome extensions (Here Vivaldi browser is your case). So as mentioned here under Communication section, Enpass App validates authenticity of both extension and Browser before making any connection and serving data for autofilling. In case of Vivaldi Browser, Extension Validity Check gets passed but the validity check for Browser gets failed and you get the Error 403. So if you turn off the code signature verification, you can bypass the Browser checking and set your foot into autofilling in Vivaldi Browser.
To avoid any security risks associated with turning off the code signature verification, please make sure that you download the browser from the legitimate source and double check the authenticity of any extension you install.
Hope it helps!