It's been there at least 6 months already.
I think I started using it around the time it was added. According to my password safe, I put the sync name/password in there on 1st of December last year.
EDIT: I want to add that I run the Snapshot versions of Vivaldi (currently 1.16.1211.3), and it could be that the Sync was introduced only in a Snapshot in December. So I have no idea if the sync can be found in the Stable releases as well. Anyways, it's under Tools/Settings/Sync tab if it's there, otherwise it isn't :)
I've used the snapshot versions as my primary browser at work and home at least since... (referring again to KeePass, checking creation date of credentials for some vivaldi related thing) ...beginning on April 2016, and for me it has worked great.
I might not be the heaviest of the heavy users (figuratively speaking :), for example I have total of 3 Chrome extensions installed (uBlock origin, some exif viewer and Google translate), so I am not aware of things that may not have worked correctly.
I used to use Opera (first 12, then the Chrome version of Opera which I didn't like), and then switched to Vivaldi when I found out about it.
i use the snapshot version as my default on mac and it's definitely usable (and then some). you do run into the odd bugginess, but anything major is usually fixed within a few days.
