You could try this:
Install Chrome. Then, close it and Vivaldi.
Delete everything in the "C:\Users\yourusername\AppData\Local\Google\Chrome\User Data" folder (or move everything in there to a safe place if you actually use Chrome and empty the folder and move it all back when you're done doing all of this).
Copy everything in "C:\Users\yourusername\AppData\Local\Vivaldi\User Data" to "C:\Users\yourusername\AppData\Local\Google\Chrome\User Data".
Start Opera, goto the URL ```opera://settings/importData" and choose to import from Chrome to see if things work.
If the above doesn't work, you can try this manual method:
Delete everything in "C:\Users\yourusername\AppData\Opera Software\Opera Stable" and "C:\Users\yourusername\AppData\Local\Opera Software\Opera Stable".
Copy everything in "C:\Users\yourusername\AppData\Local\Vivaldi\User Data\Default" to "C:\Users\yourusername\AppData\Opera Software\Opera Stable".
Copy everything in "C:\Users\yourusername\AppData\Local\Vivaldi\User Data" (except the "Default" folder) to "C:\Users\yourusername\AppData\Opera Software\Opera Stable".
Move the "Cache" folder in "C:\Users\yourusername\AppData\Opera Software\Opera Stable" to "C:\Users\yourusername\AppData\Local\Opera Software\Opera Stable".
Then, when you start Opera, if you're lucky, Opera will just use everything from Vivaldi it can.
(Always test with copies of everything in your Vivaldi profile so that if things go wrong, you can try again, and so you don't mess up your Vivaldi profile.)
