In case you are interested, I have a workaround for this.
let's say you have two workspaces, "astronomy" and "cats".
first you need to create two html files. put them in ~/.config/vivaldi and name them workspace-astronomy.html and workspace-cats.html.
in each html file insert this code:
<!DOCTYPE html>
<html>
<head>
<script>
window.onload = function() {
setTimeout(function() {
window.close();
}, 100);
}
</script>
</head>
</html>
This javascript code will close the tab/html file immediately after 100 milliseconds.
Now open vivaldi and go to the rules. here you set two rules:
if the url contains workspace-astronomy -> open in workspace astronomy
if the url contrain workspace-cats -> open in workspace cats
Here you can see that only the pattern in the filename is important for us, therefore the above js code will close the tab, since it only functioned as a placeholder.
okay, so the final step is to execute the correct command. you can test it first from command line. let say we want to open the cats workspace:
export "VIVA=/home/your-user-name/.config/vivaldi" && vivaldi --user-data-dir=$VIVA --url $VIVA/workspace-cats.html
that's pretty much it. if everything works fine than i would suggest to create a fancy .desktop file either one per workspace, or one only one .desktop file with various right-click menu options for the different workspaces.
