VNC, RDP, X11 Forwarding, WTF.

There is not a single good product to "stream" a linux desktop to a different machine without being logged in in "parallel".

They are all complicated and/or suck big time in quality.

i have currently a lightdm with xvnc running. the authentication is running inside lightdm

I can't imagine a scenario where that would be preferable. In any case, Linux and X11 are multiseat by design so that would be a rather unusual process model and therefore incompatible with a lot of tools. Unless you don't want to sign in and out using the protocol, in which case it can be a process in the user's current session, like TeamViewer.