brent I just checked on my version of Vivaldi..
Use Tab Zoom
needs to be unchecked. Then all webpages should use the same zoom.
You may want to restart the browser.. the setting didn't work for me immediately.
Once this setting is unchecked then the zoom bar in the bottom right hand corner of the browser should work for all web pages. If you change it on one then it should change every single webpage open in another tab. Also if you continue having issues I would type zoom
into the search on the settings page. It will give you a list of all Keyboard Bindings.