I've been working with Vivaldi for a couple weeks now on both Mac and Windows 10. Since there's now syncing of bookmarks yet to my knowledge, I just imported them from the browser sent to pasture, Chrome, individually. Today I open Vivaldi on the Mac and the bookmarks are all gone - not a trace of them in the trash. Interestingly enough, when I manually type in a site requiring a password, Vivaldi did remember it. To sum it up: the bookmarks have spontaneously disappeared, yet the passwords are still in the browser. On the PC, on the other hand, everything's been fine and dandy - no problem of this sort whatsoever.
That is very weird, I don't remember reading any other reports of bookmarks vanishing. The bookmarks are stored independently from the passwords, so it's not surprising that even if the bookmarks are nowhere to be found, the passwords would still be remembered.
If the bookmarks are really neither in the bookmarks section nor the trash bin within the bookmarks section, you don't have to import them individually from the PC, as you have seemed to have done before. Instead you can do "Export bookmarks" from the menu – I don't have a Windows PC here, so I can't tell you exactly where to find it, but on the Mac, it's at File->Export Bookmarks, and it should be somewhere similar on Windows.
Once you have exported all your bookmarks with that command at once into a single html file, you can copy that file to your Mac and then do "File->Import Bookmark and Settings" and choose "Bookmarks HTML file" to import all those bookmarks in one swoop.