I was able to get Vine Viewer on my powerbook connecting to Vine Server on my Mac Pro, on a local network and over the internet. And I was able to do it using SSH with password authentication.
But then I decided to make it more secure and use keys (and disable password authentication). I used SSH Helper to set this up. It works great, on my local network and over the internet. I can use Vine Viewer and Vine Server, with the keys.
The only problem - admittedly a small one - is that if I checkmark “Require SSH” in Vine Server, the VNC connection stops working. I guess it doesn’t matter much because I have set up my firewalls and port forwarding such that a port 22 connection is the only one possible (no port 5900 access to the Mac Pro), still I find it odd that I can’t select this option in Vine Server.