FIS to VFR traffic
As a PPL not an ATC, I'm not entirely sure what the original poster is asking.
You really don't have a responsibility to somebody you are not in contact with and who is going about his lawful business without any desire to talk to you..
The responsibility is to those you ARE in contact with to give them information (this needs to be carefully worded to avoid any implications of them being under ATC control and salvation as was made clear by Eva San) so they can avoid banging into the ones who are visible to you but not talking to you.
The rules in "uncontrolled" airspace are very much see and be seen .... but it is nice when somebdy with radar can see a bit further and help out.
DGG