So, in case anyone else has this problem... here's what I did - and I appreciate the advice from everyone here.
I uninstalled Vivaldi, copied my entire user data folder first though. Then, I reinstalled it. I tested it from a fresh install and LastPass worked perfectly as it should!
So, then I decided to copy my entire original userdata/default folder back into the mix.
When I did that, LastPass stopped working. So, I removed all that and re-created the default folder again (by closing Vivaldi, deleting all the files in the default folder and re-opening Vivaldi).
Then, I began copying back a few things that I really had to have. Like my bookmarks and start page setup (which I think comes with bookmarks). I also copied current session data, History, Top Sites, etc. - things like that.
When I opened Vivaldi again everything was there as it should be except for the extensions I had originally had installed and keyboard shortcuts I had customized, as well as the appearance. So, it was pretty easy to fix the last two, now I have to install the extensions.
But, LastPass did work fine with this method.
So, it's something in the default folder. No idea what. And I wouldn't bother trying to figure it out as there is more than a gigabyte of data in there!
Anyway, if you copy just the bare bones necessities from there back into your new install, it should get LastPass working again.
@poncho1962 Well, the founder of Opera (when it was Presto-based) founded Vivaldi some three years after leaving Opera. But the architecture of the two browsers is very, very different. Both are Chromium-based now, but Vivaldi has taken a totally separate path with regard to how its UI is built.