If your private network is reachable from Internet using a VPN, you probably noticed that computers do not appear in the left column of your Finder.
It does not mean that your remote computers are not available.
To connect to a Mac OSX screen sharing, from your Desktop, simply do :
cmd+K
Enter vnc://192.168.1.xxx (local ip of you remote machine)
You can do it from you terminal too :
open vnc://192.168.1.xxx