According to that, it seems that Vivaldi uses a lot of GPL'd and LGPL'd libraries, and it's not clear that Vivaldi is in compliance with the copyleft requirements of those licenses, nor is it apparent which license Vivaldi is released under as well (be it a FOSS license, some EULA, etc.).

There are also a lot of libraries listed that are licensed under the Mozilla Public License, which is incompatible with the GNU GPL. Distributing this therefore might not actually be legal, regardless of whether or not it's released under a FOSS license, since those two license families have conflicting requirements for derived works (including works which use those libraries).

YMMV, IANAL, etc., but this thing's running the risk of legal technicalities killing it in the crib.