@tchauhan I gather this is a general limitation in Chromium, the Vivaldi team would have to spend a ton of effort to modify the Chromium core to make it work and then, worse, maintain it with every Chromium update.
You can use different user profiles. Not the same thing, I know.
As for sites sharing data, I'm not sure about that (but I don't know if what I think is correct). The point people that ask for containers make is that they want to have the ability to log in to the same site using two different logins at the same time. But as far as I know, also in Chromium site A doesn't have access to the cookie jar of site b.
And of course you can go into the Vivaldi settings and disable third party cookies, and block trackers with the integrated ad and tracker blocker