user agent override-------is this for using an alternate browser, an alternate to Vivaldi?
If so, how do I fill in the user agent override field?
No it is for overriding the user agent, that is sent by Vivaldi along with each request it makes to a website.
Vivaldi is just Chrome, and will appear as Chrome to a website.
If you want the browser to say appear to a website as an iphone, you can enter an iphone user agent. To search for user agents for various devices and browsers just do a google search. They are very easy to find.