What would be the impact to the AuthN(/login/logout/other sec stuff) AuthZ if vaadin spring boot app would be configured as OAuth2 ResourceServer!? Would you be so kind to make an example?
To populate Spring SecurityContext this kind of configuration should be used...
Resource Servers are typically stateless, which a Vaadin Flow app definitely isn't. So that seems like a bit of an odd choice. Nevertheless, if you configure the Vaadin app as Resource Server, you should automatically be logged in when the token is sent. From that point on the authorization part of this add-on will work as per usual, based on the provided SecurityContext.
Since there won't be any explicit authentication like a login dialog (it's a Resource Server after all), just disable the add-ons standard authentication handling - as you also would for the interactive OAuth2 authentication: