Proton services and products do not integrate seamlessly and currently clash with each other. This is an issue across multiple services and not just a Proton VPN issue.

The issue specific to Proton VPN;
If both desktop application and browser extension is active at the same time, no connection is available in the browser (Vivaldi in this case, which offers built-in support for Proton VPN).

Solution;
The browser extension should detect if the desktop application has an active VPN connection before attempting to create its own VPN connection. Similarly, the desktop application should detect if a browser extension is currently running before attempting to create a VPN connection, and if one is found to be running, should notify the extension to disconnect from its session as the desktop application will take over the VPN connection responsibility.
Please note, the browser extension's settings should in no way be overruled by the desktop application. Settings defined for the browser extension might differ from those defined for the system-wide desktop application.

Context;
I use both the desktop application and the browser extension independent of each other. Problem is, that the VPN is unreliable and therefore I do not want it running on a system-wide level at all times, however, I generally always have it turned on in the browser extension. This becomes a problem when the system-wide desktop application has an active VPN connection, and I open my browser. I only turn on the desktop application for specific purposes, and if I then wanna open my browser I have to either turn off the desktop application or turn off the browser extension. It's obviously not the end of the world having to turn of one of the VPNs, but it's definitely a huge problem considering it's a premium paid service.

How did you forget about your own products when building them?

Warmest regards, a concerned customer, who's tired of using Proton products that fight themselves..