Update Killed Tabs
I don't know if this is just my system, but I though I'd pass along a warning. I just updated Vivaldi to 1.10.867.38 and then updated Windows (Win7 64 Pro) to 6.1.7601 Restarting Vivaldi between the updates seemed without issue (It did open an extra window for the Vivaldi site, but I closed that one first.), and I closed it by hand before restarting Windows after the Windows Update requested a reboot. When I started Vivaldi after the reboot, all my tabs were gone. This might be unique behavior because I have my "Users" folder on a separate drive from the OS, but it might be something that happens more universally. I thought I'd pass along a warning, just in case.
Did you quit Vivaldi before restarting Windows?