Don't support NetworkManager Proxy
about a Chinese Users，I open a VPN in NetworkManager，but Vivaldi don‘t use proxy with NetworkManager（Sorry，my English is bad！！！）
You can specify a proxy on the command line with something like this:
~$ vivaldi --proxy-server=127.0.0.1:80
I haven't tried it, though. It seems that the graphical user interface to specify a proxy in Vivaldi settings isn't ready yet. On Windows, Vivaldi can use the system proxy but it appears that that's not the case on Linux.