Proton don’t share nothing. You have the option to log in with the Vivaldi account, create a new account in Proton or use the existing Proton account if you have. only if you want to use it, if not, delete it and nothing had happend. It’s a mere offer from Vivaldi if you want an VPN in the browser. I anyway don’t like Browser VPN which can’t never substitute an desktop VPN, for browsers is better to use simply a proxy if you want it for, eg.country restrictions. To stay private, a VPN have to create the Tunnel BEFOR the browser goes online, this with an inbuild VPN is impossible, which can only create the Tunnel after the Browser start.