@mib2berlin Just a little update on my findings. It may help shine a little light on the workings of how workspaces are stored.
I had restored 15 pairs of Session_[number] and Tabs_[number] files and assumed they were connected to individual sessions. Today I checked the Sessions folder and found only three pairs of Session_[number] and Tabs_[number] files with my workspaces in the browser intact.
The dates may differ for all three pairs. This leads me to hypothesize that two of the pairs may be used as backups.
Apparently Vivaldi deletes workspace files that are no longer needed.
How did I found out?
Still working from the assumption the Session_[number] and Tabs_[number] files were linked to individual workspaces I decided to add a new tab to each workspace in order to change the dates on the Session_[number] and Tabs_[number] files. After that I went to the Sessions folder and noticed the above.
Eventually I decided to remove the unneeded new tabs in all workspaces. After that two Session_[number] and Tabs_[number] pairs have the same timestamp for Date modified. The other one has a timestamps of approximately 12 hrs ago.
Little lucky shot
One of the workspaces was empty. I found the files in the main workspace and moved them to the empty workspace. I had started my inquiry on the specific topic in the main workspace and had moved the tabs to a new workspace in the process. I was lucky my backup was recent enough.