Use Start Page as first tab
I have set Vivaldi to open with the start page as a blank page. When I open my first tab it is opened in a new tab and not the original Start Page tab. When I have multiple tabs open there is always a Start Page Tab open as well doing nothing. In Firefox I can set to about:blank and have that tab used as my first tab and as the last tab if all others are closed. Is this functionality available in Vivaldi?
In "settings" do you have both startpage options selected?