loosing vnc control when i run an administrator task

It sounds like the VNC server is running in User Mode and can only interact with other processes running in the same (non-elevated) context as the logged on user. As a result, VNC cannot interact with any program that requires administrative rights, which includes the Secure Desktop that's shown when User Account Control prompts for permission to start an elevated process.

You can solve this by configuring your VNC server to run as an elevated process. This might be described in a setting as Run as a service or something similar.

You could also disable UAC on the VNC host computer and be sure to log on with an account that has administrative rights, but that's an insecure workaround that you should avoid at all costs.

I'm searching a solution without success for hours and i find the solution just after my post here.

The solution is simple: you need to run the VNC server as an administrator (left click), or register the server as a service (it's an option proposed by the TightVNC installer).

Tested and approved with TightVNC server.