Did you try this? This worked for me.
In Vivaldi, open Settings, then select Calendar
Under Calendar (not Calendar Accounts), click the plus sign to add a new calendar
In
Account, select the Google account. In
Calendar Name, type a name for your tasks (I call mine "My Tasks"). Make sure Sync Calendar is
Active. Then click
Save
Create a new task; then store it in this just-created calendar (in the last entry in the task dialog box, click the down-arrow and select the just-created task calendar)
Open your Google calendar in the web version.
In the left sidebar, under My Calendar look for
Tasks and select it. (It should have a check mark next to it.)
Now test. Go back to Vivaldi and create a new task--be sure and select this just-created task calendar. Presuming that you're online and syncing, you should see this new task in your Google calendar.
If this works for you, then go back into Vivaldi's calendar settings and if you see a Default List in your calendars and any other lists, I suggest deleting them so you only have one list (the one you created above). This way, when you create a new task, the task is automatically put in your list without having to open the drop-down box and select a task list.
