I'm having the same issue. When I open the speed dial I want the address bar focused. So i.e. I press CTRL+T and just type my search query or address and press ENTER. But after that action or when I click on a page in speed dial I want the page content focused. For me this is important for having a fluent workflow for keyboard-only use cases.
So the problem seems that Vivaldi handles every tab the same. The speed dial is handled here just as a normal page. But it isn't. It is a page generated by Vivaldi and is one of the central pages.
Would be great if we could set different focus options for normal web pages and speed dial (i.e. extra option point in settings -> speed dial).