How much "Linux" the OS on the Vivaldi will be remains to be seen.
The main problem of the Vivaldi is that you can't just install your own operating system since the device is not completely run by FLOSS because to compile the FLOSS mali graphics driver [1][2] you need the Driver Development Kit (DDK) which is only available under the (apparently) proprietary ARM commercial license.
I've searched for the license text but couldn't find it anywhere. Does somebody know where to obtain it or have a copy of it to share or tell us what's written in it?

Similar restrictions seem to apply to virtually any ARM device on the market (e.g. including Raspberry PI) and for me that's a mayor show stopper.

[1] http://www.malideveloper.com/develop...ce-drivers.php
[2] http://www.malideveloper.com/develop...ay-drivers.php