New Vivaldi snapshot 1.0.279.3 is available for Windows, Mac and Linux with bookmarks sorting feature.You can sort your bookmarks both in bookmark panel and bookmark manager.
You can show bookmarks panel in the sidebar from View menu. You can access bookmarks manager from the speed dial or start page.
Sorting of bookmarks in Vivaldi browser
“You can now sort by bookmark name, nick name, address, tags or manually. To sort, in the panel use the sorting column. To sort in the manager click on the headings—highlighted in green in the screenshot. You can sort ascending, descending or with a third click reset.”