Import from FF URL-Properties
I also add notes in the properties field to the Firefox-list. Can this content be included with the import? This could fit well into the field "Description" of the Vivaldi-Folder.
If you think a feature is useful and missing in Vivaldi, please make a request.
How to make a feature request for Vivaldi
If you think your problem can be solved by a missing feature, please add a request at https://forum.vivaldi.net/category/48/features-requests in the corresponding thread for your browser version.
Add one line in bold as a short title for the feature and 2-3 lines to describe the wanted feature.